QC 23 
.P56 




Hass $£#&' 

Book ^-5"6 



FRONTISPIECE. 







THE 
ELEMENTS 

OF 



NATURAL AND EXPERIMENTAL 



PHILOSOPHY; 



INCLUDING 



PHYSICS, 

DYNAMICS, 

MECHANICS, 

HYDROSTATICS, 

HYDRAULICS, 

PNEUMATICS, 



ACOUSTICS, 

OPTICS, 

ELECTRICITY, 

GALVANISM, 

MAGNETISM, 

ASTRONOMY. 



ACCORDING TO THE LATEST DISCOVERIES, 

ILLUSTRATED BY NEARLY ONE HUNDRED ENGRAVINGS. 



BY REV. DAVID BLAIR, A.M. 



REVISED, CORRECTED, NEWLY ARRANGED AND GREATLY IMPROVED, 
AND ENLARGED, WITH MANY USEFUL AND IMPORTANT J 

ADDITIONS, ILLUSTRATIONS, EXPERIMENTS, J ." 

OBSERVATIONS, &C. 




NEW YORK: 
M'ELRATH, BANGS, & HERBERT. 

STEREOTYPED BY J. S. REDFIELD. 

1834. 



p 






Entered, according to the act of Congress, Nov. 1833, by 
m'elrath, bangs & CO. 
I* the Clerk's Office of the District Court of the Southern District of 
the State of New York. 



/«r<^ 



<« 



EXTRACTS 



FROM THE 



AUTHOR'S PREFACE. 



The Author of the following pages is aware that there already exist 
several valuable but expensive books on Natural and Experimental 
Philosophy ; and he should not have presumed to add to their number, 
except for the purpose of reducing so important a branch of know- 
ledge, in bulk and price, to the level of the business of schools, and 
adapting the whole to the present state of knowledge. 

Every instructer of youth must be aware, that mere disquisitions are 
of no use in the art of teaching ; and that no science can be taught, if 
the student does not work or perform operations in it, or answer ques- 
tions which involve the consideration of its various details. 

A grown person might read all the books which exist relative to any 
science, and yet remain totally ignorant of its principles, and be as un- 
able to apply them, or reason about them, if he did not patiently commit 
to memory its axioms, definitions, and leading divisions, and if he did 
not apply these himself to the practice of the science in which he is de- 
sirous to become a proficient. He who only reads about a science, can 
be nothing more than a smatterer ;. whilst he who commits its terms and 
elementary principles to memory, and applies them by some act of his 
own mind to the various combinations of the science, soon becomes a 
master of it. 

Such being the case with persons of mature age, how is it to be ex- 
pected that young persons, who seldom learn any thing by force of rea- 
soning, and who acquire knowledge only by mechanical means, should 
become proficients, except by committing the elementary principles of 
a science to memory, and exercising themselves in its practice ? 

In strict conformity with this principle, the " Grammar of Natural 
Philosophy" has been compiled. All the definitions and elementary 
principles have been written with a studied brevity, that they may be 
learned by rote. With these have been intermixed such easy and fa- 
miliar Experiments, Observations, and Illustrations, as will enable the 
young student to work in each science, and at the same time render its 
principles intelligible to the lowest capacity. 

If the author has not been equally happy in every department, he 
shall feel it a distinction to be honoured with the observations of intel- 
ligent Tutors, and he shall thankfully avail himself, in a future edition 
of every improvement which may be so suggested to him. 
1* 



6 PREFACE. 

He cannotlet pass this favourable opportunity to express his sense of 
the honour which has been rendered to his humble endeavours, in the 
works which he has already submitted to the public. He alludes to 
his Class Book, his Exercises in Heading, his English Grammar, and 
his Models of Juvenile Letters, in all of which he laboured diligently 
to give a popular feature to useful knowledge, and he has met with his 
rewards in the unparalleled success of his books. The letters of appro- 
bation which he has received from the heads of public seminaries, 
the almost universal adoption of these works by intelligent teachers, in 
every part of the world where the English language is taught, and the 
actual sale of nearly ten thousand copies of each of the first two books 
within a year after its publication, are testimonies, of which it has been 
the fate of few authors to boast. 

He hopes this work, the "Grammar of Natural and Experimental 
Philosophy," will not diminish the reputation he has acquired, and 
that combined with the Questions, it will be found to be the most com- 
plete course of Philosophy that has ever been submitted to Schools. 



ADVERTISEMENT 

TO THE FIRST NEW-YORK EDITION, 
BY THE EDITOR. 



Without affecting either to entertain or detain the reader with all the 
customary apologies incident to new publications. &c. the Editor of this 
Revised Edition of Blair's Philosophy will proceed at once to state the 
motives from which the present edition has been undertaken ; as the 
reader will necessarily expect something of this kind, in explanation of 
the numerous alterations and additions which have been made. These 
motives are, the reputation and scarcity of the work, the numerous in- 
quiries made for it, and the frequent complaints of the incorrectness 
and deficiency not only of the original work, but likewise of the several 
American Editions. These considerations have induced the Editor to 
venture the alterations which he has made ; and in doing so, he is fully 
impressed with the responsibility which he has assumed in making so 
many alterations in a work of considerable reputation and celebrity, in 
order however to supply our schools and academies with a book which 
might be depended upon for accuracy, simplicity, and usefulness. 

As to the plan upon which it has been conducted, it is proper to men- 
tion that, in order to rectify inaccuracies, supply defects, and to bring 
the work to a more consistent and systematic form, it was found neces- 
sary that the book should be carefully and thoroughly revised, the sub- 
jects newly arranged, a considerable portion of it recomposed, some 
things omitted, and their place supplied by selections fron\various es- 
teemed authors, and from more recent publications. In performing this 
task, the Editor conceives it to be necessary to assure the reader, that he 
has endeavoured to avail himself of every practicable means calculated to 
improve the work, and, as far as was consistent with the plan of an ele- 
mentary work, of those advantages which have been afforded him by 
the publication of new discoveries, and new works in science, besides 
some original remarks and observations interspersed throughout the 
different parts of the volume. 

In regard to the selections for the different additional articles, &c. it 
may be proper here to remark, that the composition or language of 
other authors, the Editor has not hesitated to adopt, whenever it was 
found suitable to the purpose, and in most instances, without a formal 
quotation, or reference to the particular authors ; though there is not 
perhaps a single instance in which a passage is quoted, copied, or com- 
piled verbatim. 

It is proper also that something should here be said concerning the 
manner in which this edition is presented to the public ; and, although 
the limits of the advertisement will not admit of a particular notice of 
all the alterations and additions which will be found in every part of the 
volume, yet, as some account of them will be expected in this place, it 
is presumed that the following brief notice of the most material and im- 
portant of them, will be found sufficient for the present purpose. 

In the first place then, the Introduction is wholly additional in this 
edition, and contains several appropriate definitions and explanations 



8 



ADVERTISEMENT. 



which will be useful as precognita to the work itself, and will facilitate 
the comprehension of several important propositions. Besides several 
additions to the part relative to "Matter and its Properties," and the 
"Physical Affections ol Matter," the propositions concerning the force 
of gravity at different distances from the centre of the Earth, have been 
corrected and entirely recomposed, and several new propositions have 
been introduced. Similar alterations, corrections, and additions, may 
be remarked wkh respect to the succeeding subjects, Dynamics, or, the 
Laws of Motion, Accelerated Motion, Central Forces, Centre of Gravity, 
Pendulums, Projectiles, the Mechanical Powers, &c. On all these sub- 
jects, several new, interesting, and important observations have been 
offered. 

In the same manner, all the subsequent parts have been corrected, im- 
proved, and enlarged — to the first part of the "Hydrodynamics," seve- 
ral additional definitions have been subjoined; some propositions and 
observations, which were out of place, have been transposed ; and a new 
table of Specific Gravity lias been introduced. The next part relating 
to Hydraulics, has been materially improved, and several errors have 
been corrected. A still greater number of alterations and additions 
were necessary in the part devoted to Pneumatics, particularly the first 
eight or ten articles. Besides some additional experiments with the 
air-pump, the observations relative to the phenomena of Wind, are 
wholly additional. The articles in. explanation of the barometer and 
thermometer, are greatly improved, ana a more extensive Table of the 
Effects of Heat has been given. 

The alterations, &c. in the subject of Acoustics are perhaps less nu- 
merous than in any other department of the work, though they are far 
from being inconsiderable or unimportant. 

The first part of " Optics " has been greatly improved, a few particu- 
lars have been omitted, and a better description of the Eye has been 
given. The articles relative to Microscopes and Telescopes have not 
been forgotten ; and the method of constructing the solar microscope 
has been particularly described. 

Instead of placing"Astronomy" immediately after "Optics," and con- 
cluding with Electricity, Galvanism, and Magnetism, as in the former 
editions, these subjects are arranged in their proper places, agreeably to 
the analogy of things : the propriety of concluding with Astronomy, be- 
ing sufficiently obvious. 

The division relating to Electricity required to be materially altered 
and corrected ; inconsistencies and errors are struck out ; the list of con- 
ductors and electrics is considerably increased ; and the explanation of 
atmospherical phenomena by the agency of the electric fluid, has been 
particularly considered. 

The subjects relating to " Chymical Electricity " and Magnetism have 
been somewhat enlarged, and explained agreeably to the present state 
of these sciences, at least, as far as was consistent with the plan and 
elementary character of this work. 

Of all the subjects embraced in the treatise of Blair, that relating to 
Astronomy was the most incorrect, deficient, and injudiciously ar- 
ranir'd. and required the greatest number of alterations, corrections, 
and additions. In the first place, the definition and division of the sci- 
ence, are given as additional ; so is the description of the different hypo- 
thetical systems of Astronomy ; the arguments establishing the rotund- 
ity of the Earth ; the explanation of thesigns of the Zodiac ; thesignsof the 
Ecliptic, and the relative position of the Equinoctial Points, &c. An en- 
tirely new diagram is given in illustration of these principles, and of 
the motion of the Earth, and its relative position with respect to the other 
cel^tial bodies, or the points, lines, circles, &c. of the celestial sphere 
— a subject which is very imperfectly and often erroneously explained 



ADVERTISEMENT. 9 

by astronomical writers. This remark applies with equal force to the 
phenomena of the seasons, not only with respect to the improvements 
introduced in the present volume, hut also in regard to the inaccurate 
and inconsistent manner in which some authors nave attempted to ex- 
plain those phenomena. 

The tables of the elements of the planets have been wholly expunged, 
and new ones introduced in their places, and in accordance with the 
latest and most accurate observations ; and a more particular and sys- 
tematic description of each planet will also be found in this edition. 

The precession of the equinoxes, an other astronomical obscurity 
among authors, is here explained in a satisfactory manner, by means of 
a new diagram. Several inaccuracies respecting the Moon have been 
rectified ; and the irregularities of the Moon's surface more particularly 
described. The tables of the several Satellites are mostly new, or cor- 
rected from later observations ; and the article on Comets has been 
greatly enlarged and improved. The table of the Constellations has 
been wholly omitted as being grossly deficient, or much behind the pre- 
sent state of Astronomy. The reader, however, may be referred to 
Ferguson's Astronomy, or Enfield's Institutes of Natural Philosophy, 
from which the table was copied in Blair's Philosophy. A more exten- 
sive Table of fixed stars and constellations may be seen in Ryan's New 
American Grammar of Astronomy, a work of considerable merit and 
utility. 

A more particular description of this Revised Edition is considered 
unnecessary ; and in fact, if all the alterations and improvements were 
to be particularized, an other, volume ought to be appended for that pur- 
pose. 

The Questions for Examination have been prepared and placed at the 
bottom of each page, in accordance with a plan which has always met 
the approbation of the Editor; the propriety and advantage of this me- 
thod being sufficiently apparent in its convenience, both to instructers 
and their pupils. 

The augmentations to the volume, although they compose a very con- 
siderable .part of the whole work, are mostly of a descriptive character, 
or tend to the elucidation of the various subjects in a more familiar 
manner, as well as to represent the advanced state of the science ; and it 
is hoped that they will be found such as ought, at present, to be compre- 
hended in an elementary treatise on Natural and Experimental Philo- 
sophy. Nor has any article of material importance been omitted, agree- 
ably to the prescribed limits and the plan and arrangement of the work. 

The Editor feels full confidence in recommending this volume to the 
attention of Instructers, in accordance with the manner in which it is 
now presented to the public, being well assured that it will not only 
diminish the labour of the teacher, but facilitate the improvement of the 
learner, in one of the most useful and interesting branches of human 
knowledge. 

Although the Editor has not omitted any material article that might 
serve to render the original work, in its present enlarged state, gene- 
rally acceptable and useful ; yet it can not reasonably be supposed that 
he has avoided all errors or mistakes in a work of this description ; and 
he will consider it as a particular favour to be honoured with the re- 
marks, observations, and suggestions of scientific and literary gentle- 
men. 

New- York, July 20th, 1833. 



INTRODUCTION. 



The term Philosophy is of Greek origin, and means originally 
the love of knowledge ; though it has been otherwise defined the 
knowledge of nature applied to practical and useful purposes. 

By natural things is meant all known bodies or substances, 
and the assemblage or system of them all is denominated the 
Universe. Nature, or the system of natural things which exist, 
or which are supposed to exist, have been generally distinguished 
into two great classes totally different from each other, in refe- 
rence to materiality and immateriality ; that is to say, body or 
matter, and spirit or mind. 

By observation and reflection we derive a knowledge of thevari- 
ous phenomena or appearances exhibited in the universe ; that is, 
of the properties of matter, and of the faculties and operations of 
mind. Our knowledge can never extend beyond these bounds, 
for any thing can never become known to us but by its properties, 
and we are equally ignorant of the true essence of matter and of 
mind. As all phenomena depend on properties peculiar to differ- 
ent things, so the object of Philosophy is to examine the proper- 
ties of natural things and the various phenomena exhibited in the 
universe, to investigate their causes and effects, and thence to de- 
duce such natural laws or principles, as may ultimately be applied 
to a variety of practical and useful purposes. 

On the distinction of the universal system of things into the 
two classes, matter and spirit, is founded the division of Philoso- 
phy into two great branches corresponding thereto. The first of 
these branches usually called Physics,* or Natural Philosophy,- 
relates to the properties of body or matter, the actions of natural 
bodies upon each other, and the laws or principles by which those 
actions are regulated. 

The second great branch of Philosophy, usually termed Meta- 
physics or Mental Philosophy, treats of the human mind, its fa- 
culties, passions, and operations, and of the best means for the 
discipline and improvement of those faculties. This second 
branch of science includes Ethics, or Morale Philosophy, which 

* The term Physics is derived from a Greek word, signifying natural, 
and in strictness, is not of so extensive application as the term Natural 
Philosophy. Physiology, another word of Greek origin, implying a dis- 
course on nature, is by some writers improperly considered as synony- 
mous with Natural Philosophy. 

t The adjective moral is derived from the Latin word mos, signifying 
manner, behaviour. 



12 INTRODUCTION. 

relates to the manners, the duties, and the conduct of man, con- 
sidered as a rational being. 

The science of Natural Philosophy, according to the restrictive 
definition given by sonic philosophical writers, treats of the action 
of natural bodies upon each other, considering that action as prin- 
cipal! y affecting them externally, and unaccompanied by any per- 
manent change in their interna) condition. This definition seems 
to be too limited and indefinite, as well as incorrect. It will be 
found, on reflection, that a consistent definition can not easily be 
given unless made to include natural phenomena universally. 

Natural Philosophy, according to the last definition and the 
usual acceptation of the term, is somewhat limited with respect 
to the extent or variety of the subject on Which it treats, and 
usually considered under the following subordinate branches or 
subdivisions; — Dynamics; Mechanics; Hydronamics, including 
Hydrostatics and Hydraulics, Pneumatics, Acoustics, Optics, 
Electricity, Galvanism, Magnetism, and Astronomy. Besides 
these, some writers enumerate Meteorology, Chemistry, and Phy- 
siology, as being connected in such a maimer that their mutual 
dependance requires that they should be treated under one prin- 
cipal head. Indeed, all the natural sciences are so intimately 
connected that it is not easy to trace the line of distinction, to 
define their several boundaries, or to say where the one begins and 
the other ends. They are, in fact, only the ramifications or sub- 
ordinate branches of one principal or universal science. 

The connexion between Chemistry, Electricity, Galvanism, 
Magnetism, arfcl Optics, is such as to render it altogether impro- 
per and injudicious to exclude Chemistry from the general science 
of Physics or natural Philosophy, as exhibited in most of our 
treatises on the subject, which has long remained a kind of chaos 
or confused and disorderly jumble of principles, facts, and hypo- 
theses. The study of Natural Philosophy, under its present 
form, is attended with numerous disadvantages, arising from the 
manner in which it is treated ; though it is not pretended that the 
following work has been improved to the degree here intimated ; 
however, the improvements and the alterations in the plan and 
arrangement have been made as much in accordance with these 
views as seemed practicable, without making too great a change 
in the character of a well known work, and one of confessed 
merit and utility. 

Chemistry is sometimes termed Analytical Philosophy, the epi- 
thet denoting the method of experiment usually pursued in chem- 
ical science^ the object of which, in analyzing* natural bodies, 



* The two methods of proceeding with experiments and with the pro- 
cess of reasoning, are denominated Analysis and Synthesis. Analysis, 
or the Analytic m< ta in resolving bodies or classes of bodies- 

into their component or elementary parts. Synthesis begins with the 
component or simple parts, and consists in uniting those parts so as to 
form a compound body, proceeding in the order of combination to the 



INTRODUCTION. 13 

is, to discover their component parts, or elementary principles, 
their less obvious properties, and the manner in which those prin- 
ciples are combined. 

Natural Philosophy is also denominated Experimental Philo- 
sophy, because, according to the modern method of philosophical 
reasoning, ail the conclusions and principles of the science are 
deduced from experiments, either by direct inference, or by the 
method of demonstration styled Induction. 

The method of reasoning by Induction consists in inferring 
general truths from particular facts ; or. in reasoning from the 
analogy of things by general consequence ; admitting no objections 
against the conclusion, except such as are founded on experiments 
and incontrovertible truths. The method of induction is founded 
on the belief, that things will happen in future as they have been 
known to happen in time past ; and although it is no demonstra- 
tion of general conclusions, it is nevertheless the best method of 
proof that the nature of things admits of. and may be considered 
conclusive or inconclusive, according as the induction is more or 
less general ; and if no exceptions occur from phenomena, the 
conclusion may be perfectly general. 

As to the force of evidence in this kind of reasoning, it may not 
be amiss to add the following observation ; — Mathematical truths 
only are susceptible of demonstration; conclusions in natural 
science are proved by induction; in Moral Philosophy, by moral 
argument ; and matters of fact are substantiated by credible tes- 
timony. 

The method of reasoning from experiments and observation is 
at present considered the only safe guide to real science, and has 
nearly superseded the hypothetical method of the ancient philo- 
sophers ; whose systems were mostly founded upon fanciful hy- 
potheses and far-fetched analogies, and often upon no analogy 
whatever. 

The axioms in Physics, which have been deduced from common 
and constant experience, are so evident and so generally knowu, 
that only a few of them need be mentioned in this place." 

I. A nonentity, or nothing, is not possessed of any property; 
hence, 

II. An entity or substance can not by any means be produced 
from an absolute nonentity, or from nothing*. 

III. Matter can not be 'annihilated, or reduced to nothing; or. 
the annihilation of matter is an absolute impossibility. 

Although many substances may appear to te totally destroyed, 
as for instance, by the action of fire, by evaporation, "&c. ; yet in 

knowledge of the whole ; or it consists in collecting the first principles 
into one class, according to tlieir mutual dependance, so that the princi- 
ples first in order tend always to the development of those which fol- 
low. The analytic method is'considcred preferable to the other, in con- 
sequence of being safer and more important, as well as more convenient 
in practice. 

2 



14 INTRODUCTION. 

all those cases, the substances are not annihilated, but only dis- 
persed or divided in to particles so minute and in such a manner as 

not to be discoverable by our sen 

IV. Every effect is produced by a proximate cause, and is al- 
ways proportionate to Us cause. 

The rules or maxims of reasoning in Philosophy, which have 
been formed after mature deliberation, and laid down as genuine 
in philosophical reasonings, and which must serve to prevent 
errors, as much as possible, are the following four; — 

I. Of Causes. — We are not to admit any more causes in ex- 
planation of natural phenomena, than are both true and sufficient 
to explain the phenomena. 

II. Of Effects. — Hence, to the same kind of natural effects, we 
must, as far as possible, assign the same causes. 

III. Of Properties. — Such properties as are incapable of in- 
crease or diminution, and belong to all bodies as far as experi- 
ments have ascertained, are to be esteemed the universal proper- 
ties of all bodies whatsoever. 

IV. Of Phenomena. — In experimental Philosophy, the propo- 
sitions collected from experiments by general induction from phe- 
nomena, are to be considered either as absolutely or very nearly 
true, notwithstanding any hypothesis to the contrary whatever, 
till such time as other phenomena occur, by which they may be 
either corrected, or shown to be liable to exception. 

We do not conceive it to be necessary to expatiate at large upon 
the design or utility of Natural Philosophy. Its application and 
its uses, or the advantages which the human species may derive 
therefrom, will be readily suggested to a contemplative mind by 
even a very superficial examination of whatever takes place 
amongst the bodies by which we are surrounded. The properties 
of the air in which we live, the action and power of our limbs, 
the light, the sound, the heat, and the other innumerable percep- 
tions of our senses, the action and principles of the engines and 
machinery used in Husbandry, Navigation, &c. ; the vicissitudes 
of the seasons, the variation of days and nights, the appearances 
and movements of the celestial bodies, &c. are all appropriate 
subjects for the consideration of the philosopher; and upon many 
of these depend our welfare, and even our very existence. 

Natural Philosophy is useful, because the study of it exercises 
and consequently improves the rational powers of man, and the 
intellectual faculties in general. It elevates the mind above the 
gross compasses of the body, and confirms the triumphs of mind 
over matter, enlarging the sphere of human influence over the 
various objects of the natural world, enabling man to subject them 
in the, most effectual manner to his interest or his happiness. 

Whoever wishes to make any proficiency in the study of nature, 
and to derive from philosophical studies that pleasure and that 
benefit which they are calculated to afford, should previously 
make himself acquainted with various other subjects, particularly 



INTRODUCTION. 15 

with that of Natural History, which is, in fact, most extensively 
and intimately connected with such studies. It is also obviously 
requisite that the student should previously possess some gene- 
ral acquaintance with the various branches of Mathematics, or at 
least the elements of Simple Mathematics, as Arithmetic, Al- 
gebra, Geometry, Trigonometry, and Conic Sections ; for almost 
every physical effect depends upon motion, magnitude and figure ; 
and it is not easy to estimate velocities, powers, weights, time, 
motion, &c. without a competent degree of mathematical know- 
ledge, which properly precedes and prepares the way for Natural 
Philosophy. 

Before dismissing the subject, it may not perhaps be improper 
to make a few observations concerning a w r ell known philoso- 
phical obscurity, which relates to the definition or meaning of the 
Word matter, which, in its common acceptation, is sufficiently fa- 
miliar to most persons, though its precise philosophical import is 
not so generally understood. 

The matter or substance of the bodies, which we see, feel, taste, 
or, in short, which are discoverable by our external senses.becomes 
known to us by its properties only. By these senses, we examine 
the variety of bodies existing in the world around us, and ascer- 
tain, with undoubted certainty, many of the properties which they 
possess. We know that the Sun exists, because we feel the 
warmth of its rays, and because we see its luminous and circular 
ghape; we acknowledge the existence of air, because we are sen- 
sible of the resistance it offers to the motion of other bodies, &«c. 
Now, as it is inconceivable that the Sim, the air, and all other bo- 
dies, should consist of properties only, or that properties, such as 
extension or solidity, should subsist independently, without be- 
longing to something which is extended and solid, we naturally 
infer, agreeably to the first axiom, that in all bodies there must 
be some substance to which these properties belong. This sup- 
posable substance, whatever its nature may be, of which natural 
bodies are composed, and to which all their properties belong, is 
denominated Matter. 

The term matter, although generally understood, is yet used 
with so much diversity of meaning, even among philosophers 
themselves, that a correct and consistent definition can scarcely 
be found. Some philosophical writers tell us that " Matter is the 
general term used to denote the substance, whatever its nature 
may be, of which the different bodies are composed." Others say 
" Matter is the basis of all substances, constantly changing its 
form but always maintaining its existence." Dr. Priestly has 
defined it " A substance possessed of the properties of extension, 
and of the powers of attraction and repulsion." And in Joyce's 
Philosophy," Matter is defined a solid, extended, inactive, and move- 
able substance." The term matter has no relation or reference to 
any determinate figure ; but the term body more generally implies 
some separate and determinate quantity or mass of matter. 



16 INTRODUCTION. 

Although we can discover many of the properties of matter by 
the an! of our external senses, ye 1 weare said to be perfectly igno- 
rant of the intimate nature of that matter, since we are not enabled 

to say whether it consists of particles of any definite size, shape, 
and hardness ; and whether all t lie bodies of the universe consist 
of one and the same kind of matter differently modified, or of seve- 
ral different kinds or species of matter ; and in the latter case, we 
can not form any conjecture respecting the possible or probable 
number of those different species, since the present state of philo- 
sophical knowledge does not furnish us with reasons sufficient to 
determine the number of the elements. Some philosophers have 
entertained an o inion that creru simple substance consists of a 
distinct kind of matter. Others have supposed that two or three 
primitive substances, or kinds of original matter, variously modi- 
fied and combined, constitute the materiality of the universe. New- 
ton and Sir Humphrey Davy, in whose opinion we perfectly coin- 
cide, inclined to believe tha{ the material world is originally com- 
posed of only one species of matter, and that the endless variety 
of forms, properties and phenomena, observable among natural 
bodies, may arise from diversity of form in consequence of a dif- 
ference in the arrangement of the particles of this same matter, 
and from the different relations existing between this matter and 
some etherial agent, * if any such agent exists independent of ma- 
teriality. 

The various properties of matter may be divided into two prin- 
cipal classes — General and Peculiar. General properties are those 
which belong to all bodies universally ; and Peculiar properties 
are those which belong to certain bodies only, exclusive of others. 

All matter, in whatever forms or circumstances it appears, is 
supposed to possess the following general properties ; Extension, 
Figure or Figurability, Solidity or Impenetrability, Divisibility, 
Mobility, Inertness or Resistance, Repulsion. Attraction or Gravi- 
tation, Porosity, Compressibility, Dilatibility, and Affinity. 

An essential property of matter, is that without which it is ab- 
solutely impossible for matter to have existence, as Extension and 
Figure; the others may be designated as inseparable properties. 
And material substances universally are endowed with two kinds 
of properties, Physical and Chemical ; and the study of the phe- 
nomena occasioned by these has given rise to two corresponding 
branches of science, Physics and Chemistry. The secondary Phy- 
sical properties of matter are opacity, transparency, soilness, 
hardness, elasticity, colour, density, solidity, fluidity, and the like. 



* The principles of light, caloric, electricity, and magnetism, belong 
to that class of bodies called ttherial, imponderable, unconjinabie, and 
radiant matter. And some philosophers have exposed their admirable 
iv and penetration by considering these agents as being "merely 
nanations and active powers of other substances, excited by che- 
mical agency," instead of regarding them as a distinct class of material 
substances. 



INTRODUCTION. 17 

Philosophers are not agreed as to the number of the essential or 
general properties of matter ; and in regard to one of those proper- 
ties, the "divisibility of matter," philosophical "writers are still 
so inconsistent and unreasonable as to admit the existence of 
a kind of controversy unworthy of the science, and discreditable 
to philosophical reasoning. This arises from the want of clear 
ideas concerning the meaning of certain words, used in Natural 
Philosophy, particularly the word infinity, which has occasioned 
many silly debates, the assumption of many odd positions, and 
the deduction of many absurd consequences. Such errors and 
obscurities may always be expected to exist, so long as the idea 
of something determinate is attached to the terms infinite, or in- 
finity, instead of something indefinite or indeterminate. All that 
we ought properly to understand by the word infinite, is an inde- 
terminate something, greater or less than any conceivable or as- 
signable quantity ; nor are we authorized to believe in the actual 
existence of any infinite whatever, that is, a quantity either so 
small as to admit of no farther diminution, or so great as to be_ 
incapable of farther augmentation. The idea of infinity consists, 
therefore, in the possibility of diminishing or augmenting a quan- 
tity beyond any assignable limit ; as, for instance, the infinity of 
number is simply the increase or diminution of a numerical quan- 
tity beyond any conceivable limit. A quantity infinitely small is, 
in fact, no quantity at all ; so that, in strictness, the term infinity 
does not refer to the existence of a magnitude or quantity, but to 
the possibility of the interminable increase or diminution of that 
quantity or magnitude. 

The point of dispute above-mentioned is, the "infinite divisibi- 
lity of matter," that is, whether matter is divisible in infinitum 
or not. That extension is infinitely divisible, is easily demon- 
strated ; if we understand by the term divisibility the possibility of 
being ideally divided by mathematical planes without any sepa- 
ration or discerptibility of parts ; and in this sense, matter is also 
infinitely divisible. Now, if it can be demonstrated that extension 
is divisible or separable into parts not in contact, the infinite divi- 
sibility of matter will no longer remain a subject of doubt or un- 
certainty. That matter is infinitely divisible, according to the 
preceding definition of infinity, admitting the exercise of any sup- 
posable power which does not change the nature of matter," is al- 
most self-evident, since there can be no end to this supposable or 
imaginary division. 

In connexion with this subject, it may not be improper here to 
introduce the following singular theorems of Dr. Keill, as depend- 
ing upon the infinite divisibility of matter, and derived from this 
principle. 

Theor. I. Any quantity of matter, however small, and any 
finite space, however lar^e, being given, as for example, a cube or 
sphere circumscribed about the orbit of Saturn ; it is possible for 
the small quantity of matter to be diffused through the whole 

2* 



18 INTRODUCTION. 

space, and to fill it, in such a manner, that there shall not be any 
pore or interstice in it, the diameter of which shall exceed a given 
finite line. 

Cor. Hence, the quantity of matter in any given body or sub- 
stance, being reduced into a space absolutely full, that space may 
be any given part of its former magnitude.* 

Thcor. II. Two bodies of equal bulk being given, their quanti- 
ties of matter may be unequal, in any proportion, and yet the sum 
of their pores, or the quantity of void space in each of the two 
bodies, shall be to each other nearly in a ratio of equality. 

Example. Suppose 1000 cubic inches of platina, when reduced 
into a space absolutely full, to be equal to one cubic inch ; then, 
1000 cube inches of water, which is 22 times lighter than platina, 
will, when reduced, contain 2V of an inch of matter. Conse- 
quently, the void spaces in the platina, will be to those in the 
water, as 999 to 999 %£-, or nearly in a ratio of equality. 



* This theorem and corollary may serve to elucidate or confirm a very 
singular and extraordinary opinion entertained by the great Sir Isaac 
Newton, who conjectured that if the whole mass of our planet the Earth, 
could be compressed so as to be absolutely without pores, the space it 
would then occupy would not probably exceed a cubic inch. 






THE 

ELEMENTS 

OF 

NATURAL AXD EXPERIMENTAL 

PHILOSOPHY. 



PHYSICS, OR MATTER AND ITS PROPERTIES. 

I. Matter is the general name of every- 
thing or substance, that has length, breadth, and 
thickness. 

Observation. Philosophers have in all ages discussed the general 
nature of matter, but without arriving at any satisfactory result. 
This is certain, that all we know of matter is merely relative to 
our o*vn powers and senses ; and those relative properties, being 
all we can know, are the proper objects of Philosophical inquiry. 

II. The properties of all matter or substance, 
are solidity, divisibility, mobility^, and 

INERTNESS. 

III. Solidity is that property which every 
substance possesses, of not permitting any other 
substance to occupy the same place at the same 
time. 

Illustration 1. If a piece of wood or metal occupy a certain 
space, before anything else can take possession of that space, the 
wood or metal must be removed. 

2. Water and even air have this property. 

Experiment 1. If some water be put into a tube closed at one 
end, and a piece of wood be inserted that fits the inside of the 

Questions. — 1. What is to be understood by the term Matter, as used 
in Natural Philosophy ? 2. What are the properties of all matter or 
substance 1- 3. What is Solidity ? 4. Can liquids occupy the same 
spacejwith a solid substance ? 



20 PHYSICS. 

tube very accurately, it will be impossible by any force to ^et the 
wooden piston to the bottom of the tube, unless the water is first 
taken away. 

2. The experiment may also be made with air instead of water. 

Corollary. Therefore, water, air, and all other fluids, are, with 
reference to space, equally solid with the hardest bodies. 

IV. divisibility is that property of matter, by 
which its parts may be divided and separated 
from each other. Of this division there can be 
no end. 

Ulus. 1. Since mattercan never be annihilated by division, so we 
can never imagine it to be cut into particles so small that any one 
of them shall not have an upper and under surface, which may be 
separated, if we have instruments fine enough for the purpose. 

2. It would also be absurd to say, that the greatest mass has 
more lialyes, quarters, or thousandth parts, than the smallest 
particle of matter. 

Exp. 1. If a grain of gold be melted with a pound, or 5760 
grains of silver, and a single grain of the mass be dissolved in 
diluted nitric acid, the gold, which is only the 5761st part of a 
grain, will fall to the bottom and be visible ; but the silver will 
be dissolved in the acid. 

2. A grain of gold may be hammered by the gold-beaters to 
such a degree of fineness, that the two millionth part of the grain 
may be seen by the naked eye. 

3. In addition to these experiments, we may observe, that there 
are anirnalculcE so small, that many thousands of them taken 
together are smaller than the point of a needle. Jdr. Leewen- 
hoeck informs us, that there are more animals in the milt of a 
codfish, than there are men on the whole Earth, and a single grain 
of sand is larger than four millions of these animals. Moreover, 
a particle of the blood of one of these aniinalculoe has been found, 
by calculation, to be as much smaller than a globe of the one-tenth 
of an inch in diameter, as that globe is smaller than the whole 
Earth. Nevertheless, if these particles be compared with the par- 
ticles of light, they will be found to exceed the latter as much as 
mountains exceed single grains of sand. 

4. The natural divisions of matter are still more wonderful. 
In odoriferous bodies, a surprising subtilty of parts is perceived ; 
several bodies scarcely lose any sensible/ part of their weight in 
a great length of time, and continually fill a very large space with 

4. How can it be proved that fluids can not occupy the same space oc- 
cupied by solid bodies! 5. Can water and air occupy the aame space 
at the same time I 

0. What i.s meant by Divisibility, in Natural Philosophy? 7. What 
are instances of the practical divisibility of matter to a great extent 1 
8. Give an example of the natural divisions of matter. 



OF MATTER AND ITS PROPERTIES. 21 

odoriferous particles. Dr. Keill has computed the magnitude of 
a particle of assafcetida to be only 33 trilliontbs of a cubic inch. 

5. A grain of musk is capable of perfuming for several years, a 
room 12 feet in every direction, without sustaining any sensible 
diminution of its volume or weight. But a space such as that 
above indicated, contains 1723 cubic feet, each of which contains 
1723 cubic inches, and each of these 1723 cubic lines, so that the 
number of cubic lines is the third power 1728, or 5159730352. 
It is probable that every one of these cubic lines contains some of 
the odoriferous particles ; the air of the apartment may, in the 
course of several years, be renewed 1000 times, and the grain of 
musk, without sensible alteration, may furnish new odoriferous 
particles. In calculating the tenuity of each of these, the imagin- 
ation is bewildered. • 

Corol. From all which it is evident, that matter is actually di- 
visible to a degree much greater than we can imagine ; and to 
which divisibility we can set no limits. 

V. Mobility is that property of matter by 
which it is capable of being moved from one part 
of space to an other. 

Illustration. It is found from experiment and observation, that 
all matter is capable of being moved, if a sufficient force can be 
applied for the purpose. 

VI. Inertness, or inactivity, is that property of 
matter by which it would always continue in the 
same state of rest or motion, in which it is put, 
unless changed by some external force. 

Illustration 1. It is evident that matter, as a stone, can never 
put itself in motion, unless it be in some way acted upon. 

No body or part of matter can give itself either motion or 
rest ; and therefore a body at rest will remain so eternally, unless 
some external cause puts it in motion ; and a body in motion will 
move eternally, unless some external cause stops it. 

2. Bodies in motion, as a bowl on the ground, or a cannon-ball 
passing through the air, fall from motion to a state of rest, either 
by the friction of the Earth, by the gravity or weight of the body, 
or by the resistance of the air. 

Experiment 1. A marble shot from the fingers would run but a 
small distance on a carpet; its motion would be continued much 
longer on a flat pavement ; and longer still on fine smooth ice. 
Here the friction is greatest on the carpet, and least on the ice. If 
the friction were quite removed, and the resistance of the air also, 

9. What is Mobility ? 10. In what does the property of matter, called 
mobility, consist 1 11 What is meant by the term Inertness t or Inertia 1 
12. What effect does inertness have on matter 1 



22 PHYSICS. 

the marble once put in motion, would continue in that state for 
ever. 

2. If a ball were fired from a cannon with a certain velocity, 
and if* there were do resistance from the air, it would circulate 
round the Earth perpetually, and never come to a state of rest. 

3. If a person should be standing in a boat at rest, and the boat 
be suddenly pushed from the shore, he will be in danger of failing 
backwards. And if the boat in swift motion be stopt before he is 
aware, he will fall forwards, because his tendency will then be to 
continue in the same state of motion. 

OF THE PHYSICAL AFFECTIONS OF MATTER. 

VIJ. By Attraction is meant the tendency 
that bodies have to approach each other, what- 
ever be the cause of such tendency. 

VIII. Philosophers usually enumerate five 
kinds of attractions ; as, the attractions of Co- 
hesion^ of Gravitation, of Electricity, of Mag- 
netism, and of Chemical Affinity, 

IX. The attraction of Cohesion is that by 
which the constituent particles of bodies are kept 
together. By this principle, they preserve their 
forms and are prevented from falling to pieces. 

Illustration. The attraction of cohesion takes place between 
bodies or atoms, only when they are at very small distances from 
each other. 

Experiment 1. Two leaden bullets, having each a flat surface 
of a quarter of an inch in diameter, scraped smooth and clean, 
will, on being forcibly pressed together, cohere so firmly as some- 
times to require a force or weight of nearly 100 pounds to separate 
them. 

2. Two globules of mercury, being placed near each other, will 
run together and become one large drop, — a union, which can arise 
only from their strong attraction. Drops of water will do the 
same. 

13. What would be thr* consequence, if a body were put in motion, 
and n r ) resistance should be offered 1 14. Why does not a body, once 
put in motion, always continue to move I 

15. What is me nit by th<> term Attraction ? IG. How many kinds of 

attraction are usu'illy enumprnU'd in Natural Philosophy ? 17. What is 

wer called which binds the constituent particles or atoms of bodies 

oat effect does the attraction of cohesion have on 

sohd bodies 7 19. What would be the consequence, if the power of 

cohesive attraction were utterly destroyed ? 



PHYSICAL AFFECTIONS OF MATTER. 23 

3. Two polished plates of brass, smeared with oil, will cohere 
Strongly ; and in like manner, two pieces of plate glass, though 
perfectly dry, will cohere on being laid together. 

The result of sundry experiments made by professor Mus- 
schenbroek, to show the cohesive power of different solids, may be 
seen in the following table. In estimating the absolute cohesion 
of solid bodies, he applied weights to separate them according to 
their length ; the pieces of wood which he used were parahelo- 
pipedons. each side of which was 27-100ths of an inch, and the 
metal Wxres made use of were l-10th of a Rhinland inch in diame- 
ter, and they were drawn asunder by the following weights ;— 

lb. lb. 

- Pii - - - 7 600 Copper - - - - 299| 

Elm - - - 950 Brass - - - - 360 

Alder - - - 1000 Gold - - - - 500 

Oak - - - 1150 Iron - - - - 450 

Beech - - - 1250 Silver - - - - 370 

Ash - - - 1250 Tin 49* 

Lead - - - - 29* 

X. Capillary Attraction* is reckoned a species 
of cohesion. The suspension of the fluid in ca- 
pillary tubes, appears to depend upon the attrac- 
tion of the ring of glass contiguous to the upper 
surface of the fluid ; and in capillary tubes, the 
heights to which the fluid rises are inversely as 
the diameter of the bores. 

Experiment 1. If a small glass tube open at both ends, be dipt 
in water, the w T ater will rise in the tube, higher than its level in 
the basin. The smaller the bore of the tube, the higher will the 
water rise. 

2. Procure two pieces of glass, five or six inches square, join 
any two of their sides, and separate the opposite sides by means 

20. What experiments illustrate the cohesive attraction of bodies 7 
21. What is meant by Capillary Attraction ? 22. What is tho rule 
in regard to the rise of fluid in capillary tubes 1 



' From the Latin word capillus, signifying a hair, the denomination 1 
capillary, being suggested by the more usnal mode of trying experi- 
ments on this kind of attraction, which is by means of tubes, having 
the bore about as fine as a hair. The terms, cohesion, adhesion, and ca- 
pillary attraction, are in many instances, used indiscriminately, though 
the latter two are generally used in a more restricted sense. Adhesion 
denotes that attraction which exists between contiguous particles of dif- 
ferent bodies ; and capillary attraction, that power by which fluids are 
attracted up capillary tubes. 



24 PHYSICS. 

of a small pr.ce of wax or a stick, so that their surfaces may form 
a small angle, as of 2° of a : then immerse the apparatus about 
an inch deep in a basin of coloured water, and the water will rise 
between the plates, and form a beautiful geometrical figure, or 
curve, called a. hyperbola. The heights of the several points of 
the curve above the surface of the fluid, will be to each other re- 
ciprocally, as their perpendicular distances from the line in which 
the plates meet. 

3. A piece of sugar or sponge will draw up water or any other 
fluid, upon the same principle. 

Water will ascend small capillary tubes nearly or quite to the 
top. It will also ascend the "pores of various bodies, and in fact, 
almost every porous substance ; which ascent is caused by the 
water's being attracted by that substance. 

It is probably owing to the agency of this power, that the watery 
juices are made to ascend the pores of vegetables, and also that 
water is drawn to the tops of the highest mountains, from which 
it issues in springs and flows down. 

4. Let two tubes of different bores be immersed in a vessel of 
coloured water, it will be found, that the water will rise as much 
higher in the smaller tube, as the diameter of its bore is less than 
that of the larger tube. 

XI. It is probably, owing to the various de- 
grees of cohesion, that some bodies are hard, and 
others soft ; that some are in a solid, others in a 
fluid state. 

Observation 1. As it is by the attraction of cohesion that the 
parts of a body are kept together ; so when a body is broken, it is 
this attraction' that is overcome. Hence the reason of soldering 
of metals, gluing of wood, &c. Hence also may be explained 
why some bodies are hard, others soft, and others fluid, which 
properties may result from the different figures of the particles, 
and the greater or less degree of attraction consequent thereupon. 
Elasticity may arise from the particles of a body, when disturbed, 
not being drawn out of each other's attraction ; as soon, therefore, 
as the force upon it ceases to act, they restore themselves to their 
former position. 

2. Heat has a considerable influence upon the cohesive powers 
of bodies; and, if increased to a certain degree, it will throw the 
particles of bodies out of the sphere of each other's attraction. 
Heat, or caloric, in the new language of Chemistry, is the grand 

23. How may this kind of attraction be illustrated 1 24. What natu- 
ral phenomena may be explained on the principle of capillary attrac- 
tion i 25. What effect do the various degrees of cohesive attraction 
have on bodies? 26. Why are some bodies hard and others soft 1 
27. What is the cause of Elasticity in bodies 1 



PHYSICAL AFFECTIONS OF MATTER. 25 

antagonist principle of the attraction of cohesion. These two 
opposing powers, keep nature in a state of perpetual motion. 
And it is this struggle between the contending forces of caloric 
and attraction, which prevents the extreme density which would 
result from the sole influence of the attraction of cohesion ; 
which also prevents the extreme rarity that would result, from the 
excessive influence of caloric. When the attractive force is strong- 
est, the body continues in a state of solidity ; but, if, on the con- 
trary, heat has so far removed the particles of it, as to place them 
beyond the sphere of attraction, they lose their adhesion, and the 
body becomes fluid. Thus, water, when cooled to a certain de- 
gree, becomes solid, and is called ice. Above that temperature, 
its particles not being held together, it becomes liquid; but when 
raised to a certain temperature, called the boiling point, its parti- 
cles give way to the repulsive power of fire or caloric, and flying 
off, assume an aeriform state called steam. The same may be 
affirmed of all bodies m nature, for even diamonds, the hardest 
substance with which we are acquainted, are capable of being 
dispersed by a common culinary fire. 

It may therefore be considered as an axiom, that all bodies are 
capable in certain circumstances of three states, solidity, fluidity ', 
and gas. 

XII. Repulsion is a force that is supposed to 
extend to a small distance round bodies, so as to 
prevent them from coming into actual contact.* 

Observation 1. The repelling force of the particles of a fluid is 
but small, and, therefore, if a fluid be divided, it readily unites 
again. But, if a hard substance, as glass or sealing wax be 
broken, the parts can not be made to adhere, unless they are 
moistened in one instance, or melted in the other. 

2. Sir Richard Phillips, who ascribes attraction to bodies mu- 
tually intercepting the impulse of a universal medium, acting' 
through all space, ascribes Repulsion to vortices of eddies in the 

28. What effect has heat on bodies ? 29. What two forces are said to 
act on bodies, always in opposition to each other 1 30. How may the 
influence of caloric be illustrated 1 31. In what cases may we see the 
effect of caloric in the expansion of bodies, or in the separation of their 
particles'? 32. What three states are all bodies susceptible of 7 33. 
What is Repulsion 1 34. Does this force act strongest in solids, or in 
fluids'? 



* Repulsion, or that property which enables bodies to recede from 
each other, is supposed to extend but a very small distance from the 
body, and, as some suppose, can not properly be considered a positive 
quality, except perhaps in Electricity and Magnetism. It is usually un- 
derstood to be that power which prevents bodies from closely uniting by 
the power of attraction, and is the greatest between bodies of different 
natures. 3 



26 PHYSICS. 

circumambient medium, produced by those peculiar causes which 
always accompany high degrees of repulsive action. The repul- 
sion of electricity he considers as merely relative; because every 
electrified surface has within a given distance a contrary electri- 
city, and light bodies when apparently repelled from one" surface, 
are, in truth, but attracted by the other surface : and perhaps all 
repulsion is produced by a counter-attraction. 

E x peri me nt I. Water repels most bodies till they are wet. A 
small sewing needle will swim in a basin of water 

2. Drops of water will roll on the leaves of many vegetables 
without wetting them. 

3. If a ball of light wood be dipped in oil, and put into apan of 
water, the water will be repelled from the wood, and will form a 
channel round it. 

4. Melted glass dropped into water, forms globules with a 
stem, called Prince Rupert's drops, which, on breaking the stem 
will burst with great violence, and fall into powder. 

XIII. The attraction of Gravitation, or Gra- 
vity, is the name of that force by which distant 
bodies tend towards each other. 

Observation 1. All bodies on or near the surface of the Earth 
tend towards its centre by a power called the attraction of Gravi- 
tation, or according to the writer above mentioned, by intercepted 
pressure of an elastic medium, which fills all space, and seeks to 
pervade all matter; and this seems a reasonable cause of the 
phenomenon.— Monthly Mag. Oct. 1S1I. 

2. A stone or other heavy body, let fall, will move towards tbo 
Earth till it meet with some other body to obstruct its course. And 
bodies move in lines perpendicular to the surface, because the 
point to which they ultimately tend is the centre of the Earth, and 
the line of direction produced coincides with the radius, and is at 
right-angles with the surface, which is nearly spherical. Some 
bodies ascend, because they are acted upon by a force greater than 
the attraction of gravitation, and in a contrary direction. Va- 
pours, smoke, &c. do not descend, because they are lighter than 
the air, and supported by it. 

It was indeed formerly believed that smoke , steam, etc., possess 
no weight, no gravitating powers ; but later experiments have 
shown, that these are equally obedient to the general law, as bodies 
more dense, and that it is in consequence of the density of the at- 

35. How does Sir Richard Phillips account for the power of re- 
pulsion 1 36. What simple experiments will show that bodies repel 
each other 1 37. What is Gravity, or the Attraction of Gravitation ? 
38. What causes bodies to fall to the Earth 1 39. Towards what point 
do bodies tend by this kind of attraction 1 40. What is the cause of 
this power ? 






PHYSICAL AFFECTIONS OF MATTER. 27 

mosphere, thai such light bodies ascend, or are prevented from 
falling. 

3. When we speak of attracting powers, we do not attempt to 
explain their nature or assign their causes. Having derived 
general principles or laws of nature, from phenomena, we only- 
give a name to these principles, in order to explain other ap- 
pearances bv them. 

4. The tendency of all bodies towards the Earth really results 
from their tendency towards the several parts of the Earth. For, 
by an experiment made by Dr. Maskelyne upon the side of the 
mountain Schehalhen, he found the attraction of that mountain 
sufficient to draw the plumb-line sensibly from the perpendicular. 
— See Hutton's Mathematical and Philosophical Dictionary. 

XIV. By gravity, a stone dropped from a 
height falls to the surface of the Earth ; and by- 
it the heavenly bodies are retained in their orbits. 

Observation. From this species of attraction arises all the mo- 
tion, and consequently all the mutation in the great world. By 
this, heavy bodies descend, and light ones ascend; by this, pro- 
jectiles are directed, vapours and exhalations rise, and rains fall; 
by this, rivers glide, the ocean swells, the air presses, &c. 

XY. The planets gravitate towards the Sun, 
and towards each other, as well as the Sun to- 
wards them. 

Observation. This attraction of gravitation seems to belong to 
bodies of every kind ; it is mutual amon^r them, and seems to 
pervade the universe. Experience, reasoning, and analogy, show 
that this gravitation exists not only between the globe of the 
Earth and the surrounding bodies, but between all parts of mat- 
ter. One terrestrial body gravitates or tends towards an other 
terrestrial body ; the Moon gravitates towards the Earth. The 
Moon, the Earth, and all the planets, gravitate towards each 
other, and towards the Sun ; and probably the Sun with all its 
planetary system, may gravitate towards some other object. 

XVI. By gravity all terrestrial bodies tend 
towards the centre of the Earth, and in all places 

41. Why do some light bodies, steam, smoke, etc. ascend 7 42. From 
what does the tendency of all bodies towards the centre of the Earth 
result 7 43. What experiment serves to prove the tendency of bodies 
towards the several parts of the Earth 7 44. What influence does the 
power of gravity have on the various masses of matter in the universe ? 
45. Is there any difference in the force of gravity at all places equally 
distant'from the centre of the Earth 1 



28 physics. 

equally distant from the centre of the Earth, the 
force of gravity is equal. 

Observation. See the Monthly Magazine, May 1, 1813, for an 
account of the effects of Pressure of all terrestrial substances on 
each other ; the above writer observes, that the power of pressure 
acts from the surface to the centre of all planets, or independent 
totalities of matter, necessarily and without intermission ; 
and is, or has been, the great instrument or handmaid of nature, 
nch most of its varieties of substance are, or have been 
produced. It is synonymous with the action or momentum of the 
t of bodies in their endeavour to fall to the centre of planetary 
splu-res, and with the impulse called by astronomers the princi- 
ple of gravitation. It is evidently one of the primary principles 
of nature, and would drive all atoms of matter into solid and 
immoveable contact, but for an other power called repulsion, 
synonymous to elasticity, or expansion, producing varied degrees 
oi' density. To press and to resist appear to be the conflicting 
principles or agencies, to which we may ascribe all the pheno- 
mena of nature ; and in the degree in which pressure overcomes 
resistance, or resistance counteracts pressure, heavy and light 
bodies, inert minerals, or active organizations, become the ac- 
cidents, or necessary varieties, of those active powers. To press 
and to resist appear then to be the active principles of all matter; 
or, in other words, Union by gravity, and Expansion by heat, 
seem to be the great secondary causes of all phenomena. The 
Elasticity of a universal medium producing action from icittwut, 
and substantial compression towards a centre ; and the Elas- 
ticity of heat producing action from within and Expansion 
from its respective centres, point out Elasticity as the generic 
moving power of Nature. And if Elasticity and its synonyme 
expansion, be a mere result of heat, and heat itself be merely 
a phenomenon of motion, then it would appear that motion itself, 
is the primary cause of all things ! Nor is there any incongruity 
in referring to the same primary cause, the pressure of gravity, 
and the expansion which opposes gravity, because the elasticity 
of the medium of space producing gravity is universal, and the 
elasticity producing expansion is but local and relative." 

XVII. All bodies, at a given distance, attract 
each other with a force directly proportional to 
the quantity of matter or number of ponderable 
particles which they contain. But this force, 
in bodies of a given quantity of matter, varies 
according to their distances from each other ; 

4G. Does the attraction of gravitation belong universally to all bodies ? 



PHYSICAL AFFECTIONS OF MATTER. 29 

the nearer the attracting bodies approach to- 
wards each other, the more powerful is their 
mutual attraction. 

Obs. 1. Hence, either in the attraction of cohesion or of gravi- 
tation, when the particles of any substance are sufficiently near 
to be attracted by each other, the effect of this power must con- 
tinually increase as those particles are brought by it into closer 
proximity, or as the distance between the gravitating bodies di- 
minishes. But this property or force of attraction does not in- 
crease, simply as the distances decrease ; it has been found to be 
regulated by a remarkable law, which is, that in different distances 
of the same bodies, their orce of attraction is reciprocally in the 
duplicate proportion of those distances ; that is, it decreases as 
the squares of the distances between the centres of those bodies in- 
crease^ 

For instance, if two bodies at a given distance, attract each 
other with a certain force, at half that distance they will attract 
each other whhfour times that force ; at one-third of the distance, 
with nine times that force, and so on. Or, which is the same 
thing, if a body at a given distance from an other, is attracted by 
the latter with a certain force, at twice the distance it will be 
attracted with \ of that force ; at three times the distance, with 
■9- -of that force; at four times the distance, with r^ 6 of that 
force ; and so on- * 

Obs. 2. Hence it is found that the force of gravit}*, or gravita- 
tion, is greatest at the surface of the Eaith; that bodies high in 
the atmosphere, or at a distance from the surface of the Earth, are 
acted upon by a smaller force, the farther they are removed from 
the surface ; and that the weight of a body, is less on the sum- 
mit of a mountain than in a valley beneath, &c. 

47. What is the law in regard to the gravitation of bodies towards 
each other at different distances ? 48. Is this law or prineip'e in ac- 
cordance with all ki?ids of attraction, or emanation from a centre! 49. 
Is the gravity or weight of the same body greater in some places than in 
others, and why 1 

This diminution of intensity, in the proportion of the squares of the 
distances from the Centre of Emanation, appears to take place not only 
with the force of gravitation, but likewise With all kinds of emanations 
from a centre, such as the emanations of light, caloric, sound, Arc, 
from a radiant point, as far as we are able to judge, from the pre- 
sent state of philosophical knowledge; for with the decrease either of 
sound or radiant caloric, as they are observed in nature, this law has 
not been ascertained to any great degree of accuracy. Tut, independ- 
ently of actual experiments, or known and natural phenomena under 
existing circumstances, it can be strictly demonstrated that Emanations 
which proceed in straight lines from a centre, and do not meet with 
any obstruction, must decrease in intensity, inversely as the squares 
of the distances from the centre. For it is very evident, that, for in- 
3* 



30 PHYSICS. 

XVIII. The force of gravity is less at the 
equator than it is at the poles, because the equa- 
torial diameter is several miles longer than the 
polar diameter, and because the swing, or centri- 
fugal force of the Earth at the equator diminishes 
the gravity. 

Obs. Hence, a pendulum which, in the latitude of London, 
must be 39.14 inches in length in order to vibrate seconds, requires 
to be .13 inches shorter, or but 39.0 1 inches to perform its vibrations 
in the same time at the equator. 

XIX. The force of gravity is greatest at the 
surface of the Earth, from which it decreases 
both upwards and downwarcjs ; and this attrac- 
tion or gravitating power has been ascertained 
from a variety of facts, to decrease in proportion 
as the squares of the distances from the centre of 
the Earth increase ; or, in other words, the force 
of gravity at different heights or elevations above 
the surface, is inversely as the squares of the 
distances from the centre of the Earth. 

Obs. 1. The force or attraction of gravitation, is greatest at the 
surface of the Earth, from which it decreases both upwards and 
downwards, but not in the same proportion. The force of gravity 
upwards, above, or beyond the surface, decreases as the square of 

50. Why is it necessary that a pendulum, which vibrates seconds, 
should be shorter at the equator than at the poles 1 51. Where is the 
force of gravity greatest 1 52. In what proportion does the force of 
gravitation decrease from the surface of the Earth upwards? 53. In 
What proportion does it decrease downwards? 

stance, the rays of light which thus proceed from any point on the sur- 
face of the ?5un, or from the tlame of a candle, are continually more and 
more diffused, or attenuated, and that consequently, the force or intensity 
of light diminishes more and more, as the imaginary spherical surfaces 
Increase, through which those rays are diffused at different distances. 
But these spherical surfaces arc to each other, as the squares of their 
radii, or distances from the centre, that is, from the luminous point; 
fore, thi' intensity of the rays of light, decreases inversely as the 
squares of the distances from the centre of emanation. Or, the quanti- 
ii' i of lights received from a luminous body upon a given surface are 
a* the squares of the distances of the surface from the lumin- 
ous body, fyc. 



PHYSICAL AFFECTIONS OF MATTER. 31 

the distance from the centre, but downwards simply as the dis- 
tance. At a height, for example, as far from the surface of the 
Earth, as the surface is from the centre, the force of gravity is only 
one-fourth of what it is at the surface ; for the distances being as 
1 to 2, their squares are as 1 to 4 ; and therefore the force of gra- 
vity at that height, equal to the semidiameter of the Earth above 
its surface, is to the force of gravity at the surface, as 1 to 4. 
Gravity and weight may, in particular circumstances, be consi- 
dered as synonymous terms. We say, a bod}-, as a piece of lead, 
weighs one pound, or sixteen ounces ; but if by any means it 
could be carried to the height of 3960 miles above the surface of 
the Earth, it would weigh only | of a pound, or four ounces ; and 
if it could be transported to the distance of 7920 miles above the 
Earth, which is three times the distance from the centre that the 
surface is, it would weigh only 9 of a pound, or something less 
than two ounces; at four times the distance of the surface from 
the centre, or 11330 miles above the surface, it would weigh only 
f'-g of a pound, or one ounce ; and so on. 

2. And it is demonstrated, that the force of gravity downwards, 
or towards the centre, decreases simply as the distance from the 
surface increases ; so that at one-half the distance from the centre 
to the surface, the same weight, already described, would weigh 
only one-half of a pound ; at two-thirds of the distance from the 
surface, or one-third of the semidiameter from the centre, it 
would weigh only one-third of a pound ; and in the same propor- 
tion for any other distance. 

Thus, a "piece of metal, &c, weighing, on the surface of the 
Earth, one pound, will weigh 

At tha centre of the Earth, --0 

11 990 miles from the centre, £ pound. 

" 19S0 " " £ 

" 2970 " " - I 

" 3960 " [at the surface, 1 1 

" 7920 " " | 

" 11830 " " ± 

— " 15840 " " Jg 

And at the distance of the Moon from the Earth, which is about 
236,400 miles, it would weigh only 1 -3600th part of a pound, be- 
cause the distance is about 60 times farther from the centre of the 
Earth than the surface. 

Thus it appears that a body at the centre, being equally at- 
tracted in all directions by the different parts of the Earth, would 
consequently be destitute of gravity or weight, the case of a body 

54. If a certain bodv weighs one pound at the surface of the Earth, 
what would be its weight at the centre— at 990— at 1980— at 2970— at 3960 
—at 7920— at 11330— and at 15340 miles from the centre of the Earth 1 
55. What would be the weight of the same body, when carried to the 
distance of the Moon 1 



32 DYNAMICS. 

thus situated being likewis« analogous to that of one body existing 
alone in universal space. 

It must however be recollected, that the preceding statement, 
respecting the diminution of gravity from the surface to the centre, 
is founded on the supposition that the Earth is a perfectly sphe- 
rical and homogeneous body, of equal density throughout, which 
is by no means the case, as the greater density of the Earth to- 
wards the centre has been proved both from experiments and from 
calculation. 

DYNAMICS J OR THE LAWS OF MOTION. 

XX. Motion is the continued and successive 
change of place of any body. It is absolutely 
impossible either to produce or destroy any thing 
without motion ; and every thing that happens 
depends upon motion. 

XXI. The Three Primary or General Laws 
of motion, which have been deduced from in- 
numerable experiments and observations, by 
means of the strictest philosophical reasoning, 
are as follows ; — 

i. That every body will continue in its state of rest, or of uni- 
form motion in a straight line, until it is compelled, by some force 
impressed, to change its state. 

ii. That the change of motion produced in any body is pro- 
portional to the force impressed, and is always made according to 
the direction in which that force is impressed, and, 

in. That action and reaction are always equal and contrary to 
each other. Or, the mutual actions of bodies upon each other, are 
equal and in contrary directions, and are always to be estimated in 
the same straight line. 

Obs. These three laws of motion may be illustrated by experi- 
ments, but their best confirmation arises from the circumstance, 
that all the particular conclusions drawn from them agree with 
universal experience. They were assumed by Sir Isaac Newton 
as the fundamental principles in the science of Mechanics ; and 
the. theory of all motions deduced from them, as principles, being 
found to agree, in all cases, with experiments and observations, 
the laws themselves are considered as mathematically true. 

56. What is to be understood by the term Motion ? 57. What are the 
primary laws of motion 7 



LAWS OF MOTION. 33 

XXII. We are principally concerned with 
two kinds of motion with respect to a given 
body. 

i. That motion by which an entire body is transferred from one 
place to an other ; and, 

ii. The motion of the minute parts of bodies amongst them 
selves, supposed to be the cause of fluidity and vapour. 

lllus. By the first kind of motion, a heavy body falls to the sur- 
face of the Earth, a carriage moves, and a ship sails. By the 
second, plants and animals grow, and the compositions and de- 
compositions of bodies take place. 

Exp. 1. Take a decanter of clear water, and hold it in the rays 
of the Sun, and it will be seen that the light particles contained 
in it are in perpetual motion. 

2. Let the rays of the Sun pass through a small hole in a 
window shutter, and it will be observed that the particles floating 
in the atmosphere are in constant motion, of whose existence we 
were not before aware. 

Obs. The motion of bodies may also be considered as absolute 
or relative. If we consider the motion of a body in space, with- 
out regard to that of other bodies, its motion is called absolute ; 
and the motion is termed relative when compared with that of an 
other body which is also in motion. 

XXIII. Several things require notice with 
regard to motion ;— 

1. The force which impresses the motion. 

2. The quantity of matter in the moving body. 

3. The velocity and direction of motion. 

4. The space passed over by the moving body. 
5~. The time employed in going over this 

space. And, 

6. The force with which it strikes an other 
body that may be opposed to it. 

XXIV. Every body, by its inertness, resists all 
change of state ; therefore, to put a body in mo- 
tion, there must be sufficient cause. 

58. How many kinds of motion are we principally conversant with ; 
and what are they ? 59. What are the several things to be noticed, in 
regard to motion 1 



34 DYNAMICS. 

Obs. Any body at rest on the surface of the Earth will always 
continue so, if no external force be impressed upon it to give it 
motion, and if the obstacle which binders the attraction of gravi- 
tation from carrying it towards the centre be not removed. A body 
being put into motion by some external impulse, if all external 
obstructions were removed, and the attraction of gravitation sus- 
pended, would move on for ever in a straight line ; for there would 
be no cause to diminish the motion, or to alter its direction. This 
can not be fully established by experiment, because it is impossi- 
ble entirely to remove all obstructions ; but, since the less obstruc- 
tion remains, the longer motion continues, it may be reasonably 
inferred, that if all obstacles could be removed, motion once com- 
municated to any body, would never cease. 

Illus 1. It is plain that a mass of matter, as a stone, can not put 
itself in motion ; it therefore would have for ever remained at rest, 
unless acted on by some power. 

2. When a cannon-ball is first discharged, it maybe said to 
move in a straight line ; and it is plain that this would always be 
its direction, unless some power turned its course. It is also as 
evident that it would always continue its motion forward, did not 
the friction of the air, or its own gravity, or some other cause, so 
impede its motion, as to bring it to the ground. 

XXV. The causes of motion are called motive 
powers, and are denominated muscular or me- 
chanical ; as the action of men and other ani- 
mals, the force of wind, water, gravity, the pres- 
sure of the atmosphere, or any elastic medium, 
and steam. 

XXVI. The change of motion produced in 
any body, is proportional to the force impressed, 
and in the direction of that force. 

Obs. Effects are proportional to their adequate causes. If, 
therefore, a given force will produce a given motion, a double force 
will produce the double of that motion. If a new force be impress- 
ed upon a body in motion, in the direction in which it moves, its 
motion will be increased proportionable to the new force impress- 
ed ; if this force acts in a direction contrary to that in which the 
body moves, it will lose a proportional part of its motion ; if the 

60. How may we arrive at the conclusion, that if a body were once 
put in motion, and all external obstructions removed, it would move on 
for ever in a straight line? 61. What are the causes of motion; and 
what is the power called which puts a body in motion 1 



LAWS OF MOTION. 60 

direction of this force be oblique to the direction of the moving 
body, it will give it a new direction. 

XXVII. To every action of one body upon an 
other, there is an equal contrary reaction ; or, 
mutual actions of bodies on each other are equal 
and in contrary directions, and are always to 
be estimated in the same straight line. 

Obs. Whatever quantity of motion any body communicates to 
an other, or whatever degree of resistance it takes away from it, 
the acting body receives the same quantity of motion, or loses the 
same degree o'f resistance, in the contrary direction ; the resist- 
ance of the body acted upon producing the same effect upon the 
acting body, as would have been produced by an active force equal 
to that resistance, and in the direction of it. Hence it appears, 
that one body acting upon an other, loses as much motion as it 
communicates ; and that the sum of the motions of any two bo- 
dies in the same line of direction, can not be changed by their 
mutual action. 

XXVIII. The velocity of motion is estimated 
by the time employed in moving over a certain 
space, or by the space passed over in a certain 
time. The less the time, and the greater the space 
moved over in that time, the greater is the ve- 
locity. 

Ulus. 1 . To ascertain the degree of velocity, the space run over 
must be divided by the time. 

2. To measure the space run over, the velocity must be multiplied 
by the time; for it is evident, that if either the velocity or the time 
be increased, th? space run over will likewise be increased. 

3. If the velocity be doubled, then the body will move over twice 
the space in the same time ; if the time be twice as great, then the 
space will be doubled ; but if the velocity and time be both doubled, 
then will the space be four times as great. 

_ Exam. 1. If a ship sails at the rate of 12 miles in an hour, or 
sixty minutes, then the velocity is equal to one mile in five 
minutes. 

2. If two persons set out together on a journey, and one walks 

62. In what manner do moving bodies gain a new direction J 69. What 
is meant by Reaction ; and what is its principle 1 64. How is the ve- 
locity of motion estimated 1 65. What is the general rule for calculating 
the degree of velocity in a moving body ; and for measuring the space 
run over? 



36 DYNAMICS. 

two miles and a half, and the other walks five miles, an hour, the 
velocity of the latter, will be double that of the former. 

XXIX. A body in motion must every instant 
tend to some particular point; in which case the 
motion will be in a straight line, or it may be 
continually changing the point to which its mo- 
tion is directed ; and this will produce a curvili- 
near or circular motion. 

XXX. If a body is acted upon only by one 
force, or by several forces in the same direction, 
its motion will be in the same direction in which 
the moving force acts. 

Exam. The motion of a boat, which a man at a given place 
draws to him with a rope, is of this kind. 

XXXI. Equable motion is either simple or 
compound. Simple motion is that which is pro- 
duced by the action, or impressed force, of one 
cause. Compound motion is that which is pro- 
duced by two or more conspiring powers, that is, 
by powers whose directions are neither opposite 
nor coincident. 

XXXII. If two or more forces, differently di- 
rected, act upon the same body at the same time, 
as it can not obey them all, it will move in a di- 
rection somewhere between them. This is called 
the Composition and Resolution of Motion. 



66. In what manner is curvilinear or circular motion produced? 67. 
In what direction is the motion of a body which is acted upon by one 
force only, or by several forces in the same direction? 68. What is to 
be understood by Sunple and Compound motion? 69. What is to be 
understood by the Composition and Resolution of Motion 1 



LAWS. OF MOTION. 37 



Fig. 1 




Ulus. Suppose a body a to be acted upon by 
an other body in the direction a &, while at the 
7^ same time it is impelled in the direction a c, 
then it will move in the direction a d. If the 
lines a b, and a c be made in proportion to the 
forces, and the lines c d, and d b, be drawn 
parallel to them, so as to complete the paral- 
lelogram, then the line which the body a will 
describe, will be in the diagonal a d, and the length of this line 
will represent the force with which the body will move. 

Exam. 1. There are many instances in nature, of motion pro- 
duced by several powers acting at the same time. A ship driven 
by the wind and tide is one ; so also is a paper kite, acted upon 
by the wind in one direction, and by the string in an other. 

2. A ball fired from a cannon is acted upon by two forces, the 
one is that occasioned by the powder, the other is the force of 
gravity. 

XXXIII. The force or power of overcoming 
resistance, in any moving body, is as its momen- 
tum, or quantity of motion. 

Obs. Since a body having a certain decree of motion is able to 
overcome a certain degree of resistance, it is manifest, that with 
an increased momentum, it will be able to overcome a greater re- 
sistance. Hence the momentum of any body is measured by its 
power of overcoming resistance. 

XXXIY. In moving bodies, if the quantities of 
matter be equal, the momenta, or quantity of mo- 
tion, will be as the velocities. 

Obs. If the body A be equal to the body B, but A has twice the 
velocity of B, A has twice as much motion as B. 

XXXV. The velocity of two bodies being 
equal, their momenta will be as their quantity of 
matter. 

Obs. If the bodies a and b. Fig. 1, move with equal velocities, 
since every portion of matter in a has as much motion as an equal 

70. How is this kind of motion illustrated 7 71. What is the propor- 
tion between the force or power of a moving body, and its momentum'? 
72. Illustrate this law. 73. What is the law in regard to the force of 
moving bodies, and their momenta, when the quantities of matter are 
equal, and their velocities unequal ; and when the velocities are equal, 
and quantities of matter unequal ; and also when the quantities of mas- 
ter and the velocities are unequal ? 
4 



38 DYNAMICS. 

portion of 6, it is evident, that if a has twice the quantity of mat- 
ter of b, it must have twice as much motion. 

XXXVI. The momenta of moving bodies, 
are in the compound ratio of their quantities of 
matter and velocities. 

Obs. The greater quantity of matter there is in any body, and 
the greater velocity it moves with, the greater will evidently be its 
quantity of motion, and the reverse, it, for example, the body A 
be double of the body B, and moves with twice its velocity, the 
momentum of A will be quadruple of that of B; for it will have 
twice the momentum of B from its double velocity, and also twice 
the momentum of B from its double quantity of matter. Hence, 
if in two bodies the product of the quantities of matter and veloci- 
ties are equal, their momenta are equal, or as tho products. 

OP ACCELERATED MOTION. 

XXXVII. Accelerated motion is that in 
which the velocity is continually increasing from 
the continued action of the motive power. Uni- 
formly Accelerated motion, is that in which the 
velocity increases equally in equal times. 

IUus. 1 . The increasing velocity with which a body falls to the 
Earth, is an instance of accelerated motion, which is caused by 
the constant action of gravity. 

2. A cannon ball is acted on by a single impulse of the powder 
and the accelerating force of gravity, it therefore describes a curve. 
This is the foundation of the art ot gunnery. 

Obs. Were falling bodies moved only by one impulse from at- 
traction, the power of gravity not continuing to act on them dur- 
ing their descent, they would fall, from whatever height, with the 
same equable or uniform motion through their whole course, pass- 
ing through equal spaces in equal times; but falling bodies do not 
move in this manner ; they fall with accelerated, that is, continu- 
ally increased velocities, occasioned by the constant action of 
gravity, which, by adding a new impulse at every instant, gives 
an additional velocity every particle of time. 

XXXVIII. A new impression being made 
upon a falling body, at every instant, by the con- 

74. When are the momenta of moving hodies equal 1 75. What is 

Accelerated motion 1 76. What is meant by Uniformly Accelerated 

motion ? 77. What familiar instances may be given of this kind of mo- 

78. What causes acceleration of motion 1 79. Why does a ball, 

Bhot out of a cannon, describe a curve 1 



OF ACCELERATED MOTION. 39 

tinued action of the attraction of gravitation, and 
the effect of the former still remaining, the velo- 
city must continually increase. 

IUus. Suppose a single impulse of gravitation, in one instant, 
to give a failing body one degree of velocity; if, after this, the 
force of gravitation were entirely suspended, the body would 
continue to move with that degree of velocity, without being ac- 
celerated or retarded. But, because the attraction of gravitation 
continues, it produces as great a velocity in the second instant, as 
in the first ; which being added to the first, makes the velocity in 
the second instant, double of what it was in the first. In like 
manner, in the third instant, it will be tripled ; quadrupled in the 
fourth ; and in every instant one degree of velocity will be added 
to that which the body had before ; that is, the motion will be uni- 
formly accelerated. 

XXXIX. Motion is said to be retarded, if its 

velocity continually decreases ; and to be uni- 
formly retarded, if its velocity decreases equally 
in equal times. 

Obs. The motion of a stone or other heavy body thrown up- 
wards, is an instance of retarded motion, which kind of motion is 
always produced by some force acting upon the body in a direc- 
tion opposite to that which first put it in motion. 

XL. The velocities of falling bodies, are in 
proportion to the spaces run over, and the spaces 
passed over in each instant, increase as the odd 
numbers 1, 3, 5, 7, 9, &c. a Fig , 2 . 

illus. The space described by a body ^ "\. 
falling from a Fig. 2. in the time ex-y- ^^\^ 

pressed by a o, with a uniformly accel- b X 

erated velocity, represented by the lines 
d e,on which the last degree is express- 
ed by b c, will be represented by the £ 
area of the triangle a b c. If gravity 
ceased to act, the space passed over in 
the next portion of time b f, would be f _ 
measured by b f, multiplied into the ve- DC 



80. In what manner may the acceleration of motion in a falling body 
be explained 1 81. What is retarded motion? 82.. How is retarded mo- 
tion produced 1 83. What is the proportion between the velocities of 
falling bodies and the spaces passed over. 



40 DYNAMICS. 

locity b c, that is by the rectangle b c gf^ which is cqnal to twice 
the triangle ab c. But if gravity still acts, then the triangle eg 
h must be added ; of course, the Wly moves over three times the 
space in the second instant that it did in the first. The next por- 
tion of time it would move over five times the space represented 
by the two rectangles and triangle ; and in the fourth portion of 
time, seven times ; and so on in arithmetical progression. 

It follows, that the whole space described, is as the square of 
the time ; that is, in twice the time it will fall through four times 
the space ; in thrice the time, nine times the space, &c. 

7?/?/^. The time of a descent of a 
falling body being represented by any 
portion A B of the base of a triangle, 
the velocity will be proportional to 
B C, which is equal to A B, and the 
space described during the time D E, 
supposed infinitely short, will be 
proportional to the area D E F G, 
which is expressed by the product 
B C, and D E ; consequently the 
whole area A E F, will represent 
the space described in the time A E 
A: V JiXi -TJ.ar.d A H L the space described in 

the time A. HI but A H I is half of the square « K, and A E F of 
E L ; the space is therefore always as the sguare of the time, and 
is equal to half the space which would be described in the same 
time with the final velocity. 

Obs. All bodies descending in vacuo, by their own gravity, are 
found to fall through 16.1 feet in one second, and to acquire a ve- 
locity in falling, which would carry them uniformly through 32.2 
feet in the next second ; and an increase of velocity, equal to this, 
is found to be added to every succeeding second of time. 

2. In the first instant there is one space run through ; at the end 
of the second, there are four ; at the end of the third, nine ; and 
so on. 

XLI. It has been found by experiment, that a 
body falling from a height, moves at the rate of 
about 16 feet in the first second of time ; in the 
next 48, in the third 80, in the fourth 112 feet, 
and so on. 



C TV 


/ 

/ 


/ 


G/ 





8-1. How may the laws of falling bodies be illustrated bv means of dia- 
pram^ I is the law of descent in regard to bodies descending 

in a vacuum. 86. Through what space will a heavy body, suspend 
air, descend, during the first second of time I B7. What space the next 
second 7 88. What space in the third second of time. 



OF CENTRAL FORCES. 41 

Exam. The space will therefore be 16 in the first second; 
IQ _l_ 4g or 64 = 16X4; 4 being the square of 2, in the second 
second ;'l 6'+ 43 + 80, or 144 = to 16 X 9; 9 being the square 
of 3, in the third second ; 16 + 48 + 80 + 112, or 256 = 16 X 
16 ; 16 being the square of 4, in the fourth second. And so on, 
because 4, 9, 16, &c are the squares of 2, 3, 4, &c. 

XLIL The force with which a body moves, 
or which it exerts upon an other body, is always 
in proportion to its velocity multiplied by its 
weight, and this force is called the Momentum 
of the body. 

Jllus. If two equal bodies move with different velocities, their 
forces. or momenta are in proportion to their velocities. 

Exp. 1. If two equal cannon-balls be projected by different 
quantities of powder, so that the velocity of the one is double that 
of the other, then the force or momentum of the former will be 
double that of the other. 

2. If two stones, one of two pounds, and the other of six pounds, 
be hurled with equal velocities, the force or momentum of the lat- 
ter will be three times greater than that of the former. 

Corol. In all cases, the momenta of bodies must be as the quan- 
tities of matter multiplied into the velocities. 



OF CENTRAL FORCES. 

XLIII. All motion produced upon a body, by 
one force only, must be made in the same straight 
line. 

Therefore, a body moving in a curvilinear direction must be 
acted uoon by two forces at least ; and when one of these ceases 
to act, trie body will move again in a straight line. 

89. At what rate are the velocities of falling bodies increased 1 90. 
What is the rule tor estimating the force of a moving body 1 91. What 
is the Momentum of a body ; and of what is it composed % 92. Illustrate 
the rule for computing the momentum of a moving body. 93. What is 
meant by Central Forces? 94. What is essential to the production of 
curvilinear motion % 95. When a body is acted on by one force only, in 
what direction does it move 2 



* The doctrine of central forces may perhaps be studied with more 
advantage in connexion with Astronomy, and may be passed over by 
the junior student in this place, particularly if he has not learned the 
first six books of Euclid ana the first four rules of Algebra. 
4* 



42 DYNAMICS. 

IIlus. A stone in a sling is moved round by the hand, while it is 
polled towards the centre of the circle which it describes, by the 
string. But when the string is let loose, the stone flies oft' in a 
tangent to the circle. 

Obs. The admirable doctrine of curvilinear motion, deserves the 
greatest attention of the philosopher, since it unfolds the grandest 
phenomena of nature, and accounts in a simple and beautiful man- 
ner for every particular relative to the motions of the celestial 
bodies. 

XLIV. The force which impels a body to- 
wards a centre when it revolves in an orbit, or 
circle, is called the Centripetal force ; that by 
which it endeavours to recede from the centre, is 
called the Centrifugal force ; and these com- 
bined forces are called Central Forces. 

Obs. 1 . It is a consequence of the first law of motion, that a body, 
which revolves about a centre, must of itself, in every point of its 
orbit, tend to move in a tangent to that point ; and because all the 
points of the tangent are more remote from the centre than the 
corresponding points of the curve, it is evident that the body must 
endeavour, in every point of its orbit, to fly off from the centre of 
motion. This endeavour, which is called a Centrifugal force, is 
opposed to the Centripetal force, which, by drawing the body to- 
wards the centre, continually retains it in the curve. But it has 
been improperly stated, that the projectile and centrifugal forces 
differ from each other as the whole from the part ; as these forces 
are dissimilar in kind, and are incapable of comparison, having 
the same relation to each other as pressive and percussive forces. 
The centrifugal force is rather a consequence than a part of the 
projectile force. 

2. When bodies revolve in a circular orbit 
about a centre, the centripetal and centrifugal 
forces are equal ; because the periphery of the 
circle is in the middle between the place where 
the body would have been if it had moved in a 
straight line either towards the centre or in the 
tangent. If a body revolve in the circle b d, Fig. 
4, in the time in which it describes the arc b n, 
it will have been impelled towards the centre, 

96. Of what importance is t lie doctrine of curvilinear motion? 97. 

What is understood by Centripetal Force 1 . 98. What is understood by 

''// % al Woree . ? 99. What relation do these two forces have to each 

' 100. When bodies revolve in an orbit, what proportions do the 

centrifugal and centripetal forces bear to each other 1 







OF CENTRAL FORCES. 43 

throu^the space a n, for, by the projectile force alone, it would 
have been carried from b to a. The line a n is then the space de- 
scribed by means of the centripetal force, and this force is propor- 
tional to a n. But if, when the body was at 6, no centripetal force 
had acted upon it, instead of describing the arc b nit would have 
moved along the tangent b a, and the line n a would have been 
the space through which it would have parted from the centre ; 
therefore the centrifugal force is proportioned to n a. Both these 
forces being then proportional to the same line n a, they are 
therefore equal to each other ; and in fact, these forces are in all 
cases equal and contrary to each other. 

XLV. In all determinate orbits, described by 
bodies revolving with certain velocities in non- 
resisting mediums, about a centre of attraction, 
the areas, which are described by a straight line 
connecting the centre of attraction and the re- 
volving body, are situated in one invariable 
plane, and are always proportional to the times 
in which they are described ; or, equal areas are 
described in equal times ; and in unequal times, 
areas proportional to the times. 

Obs. 1. The velocity of a body revolving freely about an im- 
moveable centre, is inversely, as a perpendicular let fall from that 
centre on a straight line that touches the orbit. For the veloci- 
ties will be as the lines moved over, which being the bases of 
equal triangles, must be inversely as the heights of the triangles ; 
therefore the velocities are inversely as the heights, which are 
measured by perpendiculars, let fall from the common centre. 

2. When a body describes equal areas in equal times, about an 
immoveable point, or proportional areas in unequal times, it is 
impelled towards that point by the centripetal force, which re- 
tains it in its orbit. 

3. The planetary bodies of our solar system revolve in accord- 
ance with this grand law of nature ; and as will be shown here- 
after, the squares of their periodical times are as the cubes of their 
distances from the common centre of attraction, which is very 
near the centre of the Sun; and thus Sir Isaac Newton's theory of 
mutual and universal attraction amongst the bodies of the uni- 
verse is shown to be so consonant with the strictest mathematical 
reasoning, and with all the celestial appearances, that none but 

101. Illustrate this relation by means of a diagram 1 102. What is the 
law in regard to bodies revolving in determinate orbits, and describing 
equal areas, and unequal areas 1 



44 DYNAMICS. 

the ignorant and vulgar can refuse tlieir assent to his admirable 
theory. 

XLVI. The centripetal forces of bodies, re- 
volving in different circular orbits about the same 
centre towards which they tend, are as the 
squares of the arcs described in the same time, 
divided by the radii of the circles. 

Carol. Thereforethe centripetal forces of equal bodies revolving 
in circular orbits, are as the squares of the velocities directly, and 
as the radii of the orbits inversely. 

Illus. Because the length of arcs described in the same time, 
must be in the proportion of the velocities, and the centripetal forces 
are as the squares of the arcs described in the same time, divided 
by the radii ; these forces are also as the squares of the velocities 
divided by the radii ; that is, as the squares of the velocities di- 
rectly, and the rami of the orbits inversely. Hence the centri- 
petal forces of equal bodies, at equal distances from the centre, 
are as the squares of the number of revolutions in any given 
time ; for this number is as the velocity with which the body 
moves. 

XL VII. The centripetal forces of equal bodies 
revolving in equal circular orbits, are inversely 
as the squares of their periodical times. 

Obs. The circular orbits or spaces being equal, the times in 
which these are described, or the periodical times, are inversely as 
the velocities ; and therefore, the squares of the periodical times 
are inversely as the squares of the velocities, or the squares of the 
velocities are inversely as the squares of the periodical times; 
but the centripetal forces are as the squares of the velocities ; 
therefore, these forces are inversely as the squares of the period- 
ical times. 

XL VIII. The centripetal forces of equal 
bodies, revolving in unequal circular orbits, if 
the periodical times are equal, are as the radii of 
the circles 

103. How are the centripetal forces of bodies, revolving in different 
circular orbits, about the same point, determined? 104. In what man- 
ner may we determine the centripetal forces of equal bodies, revolving 
in different circular orbits ? 105.* What is the rule for determining the 
centrifugal forces of equal bodies revolving in unequal circular orbits, 
their periodical times being equal? 






OF THE CENTRE OF GRAVITY. 45 

XLIX. The centripetal forces of equal bodies, 
revolving in circular orbits, are as the radii of 
the orbits, or distances, directly, and as the 
squares of the periodical times inversely. 

Mus. If the periodical times are equal, and the radii unequal, 
the force is as the radii. If the radii are equal, and the periodical 
times unequal, the forces are inversely as the squares of the peri- 
odical times. Therefore, if both the radii and periodical times 
are unequal, the forces will be in the compound ratio of both, or 
as the radii directly, and the squares of the periodical times in- 
versely. 

L. When bodies revolve round the same 
centre, if the squares of their periodical times 
are as the cubes of their distances from the centre, 
the centripetal forces will be inversely as the 
squares of their distances. 

LI. The centrifugal forces of revolving bodies 
are as their quantities of matter. 

For the whole centrifugal force of any body is made up of the 
centrifugal forces of each particle of matter 01 which it consists ; 
and therefore, the more numerous the particles of matter in any- 
body are, the greater will be its centrifugal force. 

OF THE CENTRE OF GRAVITY. 

LTI. The Centre of Gravity of a body, is that 
point about which all its parts do in any situa- 
tion exactly balance each other, so that if a body 
be suspended or supported by the centre of 
gravity, it will rest in any position. 

106. What is the relation between the centripetal forces of equal bodies, 
revolving in different circular orbits, their periodical times being un- 
equal ? 107. What is the relation between the centripetal forces of 
bodies revolving round the same centre, if the squares of th?ir period- 
ical times are as the cubes of their distances from the centre 1 108. 
What is the proportion between the centrifugal forces of revolving 
bodies and their quantities of matter 7 109. What is the Centre of 
Gravity of a body 1 



46 DYNAMICS. 

LIII. Whatever supports the centre of gravity, 
bears the weight of the whole body; therefore, 
the whole weight of a body may be considered 
as balanced in this point. 

LIV. The common centre of gravity of two 
or more bodies, is the point upon which they 
would rest in any position. 

Fig. 5. 

. Illus. If the centres of gravity of two bodies, 

a G g AB, Fig. 5. be connected with the straight line 

^"C AB, the distances AC, and BC, from the com- 

mon centre of gravity, C, are inversely as the weight of the bodies 
A and B ; that is, the point of C will be as much nearer to A than 
to B, as A is heavier than B ; that is AC : BC :: B : A. 

Exp. Suppose A to be a ball of 12 pounds, and B to weigh 
only 4 pounds, and the length of AC, to be five inches ; then BC 
will be 15 inches ; for it will be, 5 : BC :: 4: 12, or 4 X BC = 5 

X 12 = 60, and BC =- 60 -~ 4 = 15. 

Obs. If the centre of gravity of three or more bodies is required, 
it may be found in the same manner— by first finding the centre of 
gravity of any two, and considering that as the centre of gravity 
of one body, of which and the third body the centre of gravity 
may be found the same as before ; and so on, for any number of 
bodies. 

LV. The Centre of Motion is the point about 
which the body moves ; and a heavy body sus- 
pended on a centre of motion will be at rest, if 
the centre of gravity is directly under, or above, 
the centre of motion. 

110 What part of a body must be supported to keep it from falling 7 
111. What is the common centre of gravity of two or more bodies'? 112. 
How can the centre of gravity be found 1 1 13. Illustrate, by means of a 
diagram, the manner in which the centre of gravity of two bodies can 
be found 1 114. How can the centre of gravity of three or more bodies 
be determined 1 1 15. What is the Centre of Motion 1 



OF THE CENTRE OF GRAVITY. 



47 



Fig. 6. 




lllus. If a heavy body E, Fig. 6, 
hang's by a string on a centre of mo- 
tion C, the action of gravitation at E 
is in the direction EL, contrary to the 
direction in which the string acts to 
^A. prevent the body from falling;. In this 
position, therefore, the opposite forces 
being equal in contrary directions, 
destroy each other, and the body is at 
rest. But if the body is at P, one of 
the forces acts in the direction PC ; 
and the other in the direction PL, that 
is, in a direction oblique to each other, 
whence the body will move in the 
diagonal of the parallelogram formed by PC, PL. And since in all 
cases, without the aid of mechanical powers explained hereafter, 
the force which sustains any body must be equal to its weight, the 
centre of gravity can only he at rest when these forces are in the 
same line of direction, that is, when the centre of gravity is di- 
rectly under, or directly above the centre of motion. 

LVI. If a line be drawn perpendicular to the 
horizon, from the centre of gravity of a body, it 
is called the Line of Direction, because it is the 
line which the centre of gravity would describe 
if the body were suffered to fall. 

LVI1. While the line of direction falls within 
the base upon which the body stands, the body 
can not fall ; but if it fall without the base the 
body will tumble. 

Fig. 7. 

Mus. The inclined body abed, Fig 7. 
whose centre of gravity is e, stands firmly, 
because the line of direction ef, falls within 
the base. But if the body abgh be placed 
upon it, the centre of gravity will be raised 
to Z, and then the line of direction Id will 
fall out of the base ; of course, the centre 
of gravity is not supported, and the whole 
must fall. 







Obs. This proves the injurious effect of 
rising in a coach or boat in danger of over- 
setting, the centre of gravity being thereby 
raised, and the line of direction thrown out 
[Questions on the next page.] 



48 DYNAMICS. 

of the base. Whereas, in such circumstances, the proper course 
is to lie clown in the bottom, so as to bring the line of direction, 
and consequently the centre of gravity, within the base, and 
thereby remove the danger of oversetting. 

LVIII. The broader the base, and the nearer 
the line of direction is to the centre of it, the 
more firmly does a body stand ; and the 
narrower the base of a body, and the nearer the 
line of direction is to the side of it, the more 
easily it is overthrown. 

Obs. Hence a sphere is easily rolled along ; and a narrow 
pointed body is with difficulty made to stand. 

LIX. If a plane be inclined on which a heavy 
body is placed, the body will slide down upon the 
plane, while the line of direction falls within the 
base ; but it will roll down, when that line falls 
without the base. 

Fig. 8. 

Ilhis. 1. The body e, Fig. 8. having 
the line of direction ea within the base, 
will only slide down; but the line of 
direction ba of the body b falling out 
of the base, that body rolls down the 
plane. 
2. When the line of direction falls 
-, within the base of our feet, we stand ; 
ct and most firmly, when it is in the 
middle ; but when it is out of the base, 
we fall unless Ave step out, and this is the principle of walking. 

3. Rope-daucers are able to perform their feats by knowing how 
exactly to keep the common centre of gravity, of themselves and 
their pole, just within the extended base. 

116. Under what circumstances will a heavy body, suspended on the 
centre of motion, be at rest ; and why ? 117. How may this be illustrated 
by the diagram? IIS. What is meant by the Line of Direction ? 119. 
Where must the line of direction come to prevent a body from falling? 
120. Explain this by means of a diagram. 121. What inference of 
practical utility may be drawn from this illustration? 122. Why is a 
heavy body more apt to keep an upright position, when placed on a broad 
base, than on a narrow one ? 123. Why do spherical bodies roll down a 
Blope or inclined plane ? 124. How can the motion of a heavy body be 
explained, when it slides down upon an inclined plane, and also when it 
rolls down ? 




OF PENDULUMS. 49 

4. We tipply this principle in the common actions of life ; thus, 
we bend our bodies forward when we rise from a chair, or go up 
stairs ; — so a man leans forward when he carries a burden on his 
back ; and to the right and left, as he carries it on the opposite 
side. 

OF PENDULUMS. 

X. A Pendulum is a heavy body hanging 
at the end of a string or a wire, which is move- 
able at a centre or fixed point of suspension ; and 
each swing, or the whole motion of the pendu- 
lum one way, is called a Vibration or Oscillation. 

Obs. The vibrations are produced by the falling of the weight 
to the lowest part of the circle, and by the force acquired in the fall. 

LXI. All the vibrations of the same pendu- 
lum, whether great or small, are performed in 
equal times; and the longer a pendulum, the 
slower are its vibrations, the squares of the times 
being inversely as the lengths. 

LXII. A pendulum that vibrates in the latitude 
of London in a second of time, is about 39.14 
inches in length ; but a pendulum that vibrates 
seconds at the equator, must be only 39.01 inches. 

Obs. With respect to the lengths of pendulums which vibrate 
seconds iu the same latitude, most writers on Natural Philosophy 
differ, since the length must be determined b}- means of experi- 
ments. Thus it appears that the length of a pendulum which 
vibrates seconds in or near the latitude of London, is differently 
stated bv different writers — at 39.1 inches; 39.1196; 39.125; 
39.128; 39.130; 39.14; 39.2; 39.278; 39.37079 ; &c* 

125. How is it that rope-dancers are able to. perform their feats of agi- 
lity without falling? 126. Why do we bend our bodies forward when 
we rise from a chair, or ascend a hill ; and backward, when we descend 
a hill, or a flight of stairs ? 127. What is el Pendulum? 12S. What is 
a Vibration or Oscillation : and how are the oscillations of a pendulum 
produced? 129. What is the rule for calculating the times in which 
pendulums of different lengths perform their vibrations 1 130. What is 
the length of a pendulum that vibrates seconds in the latitude of Lon- 
don*? 131. Why must it be shorter at the equator, to vibrate in the 
same time 1 

• The length 39.1L96 was ascertained by Mr. John Whitehurst, F.R.S., 
5 



50 DYNAMICS. 

IA'111. A pendulum, to vibrate once in half a 
second, must be one-fourth part as long, as one 
that vibrates seconds; and a pendulum to vibrate 
once in two seconds, must bo four times as long, 
as one which vibrates seconds ; and so on, agree- 
able to Article 61. 

Obs.l. As it is found by experiment that a pendulum which 
vibrates 00 times in a minute, is 39. 14 inches nearly, therefore to 
find how long pendulums must be to vibrate 30, 50, and 120 times 
in a minute, we say, 

Inches. Inches 

as 30- : 60* : : 39.14 : 156.56 

60 8 : 60 2 : : 39.14 : 56.36 

120 2 : 60 2 : : 39.14 : 9.78 

2. As heat expands and cold contracts all metals, a pendulum 
rod is longer in warm weather than in cold ; and irregularity hence 
takes place in clocks. 

3. The vibrations of pendulums are subject to many irregulari- 
ties, for which no effectual remedy has yet been devised. These 
are owing partly to the variable density and temperature of the 
air, partly to the rigidity and friction of the rods by which they are 
suspended, and principally to the dilatation and contraction of the 
materials of which they are formed. The metal rods of pendu- 
lums are expanded by heat, and contracted by cold ; therefore, 
clocks will go slower in summer and faster in winter. The com- 
mon remedy for this inconvenience, is the raising or lowering of 
the bob of the pendulum by means of a screw. 

PROJECTILES. 

LXI V. Bodies thrown horizontally or obliquely 
into the air, have a curvilinear motion, and the 
path which they describe is the curve, called a 
Par abola* 

132. How long must a pendulum be trt vibrate half seconds nt Lon- 
don.' 133. What is the length of a pendulum which vibrates once in 
two seconds at the same place 1 134. Mow long must a pendulum be to 
vibrate ten times in a minute at London'? 135. What is the reason that 
ister in winter than in summer; and how is this irregularity 
usually remedied ? 136. What is the path described by a body thrown 
horizontally or obliquely into the air 1 



Ul the year 1786; and the last determination 39.37079, has been more re- 
cently "ascertained by Capt Kater.— See Phil. Trans. 1818. 



OF THE MECHANICAL POWERS. 51 

Obs. 1 . A projectile is any moving body which has received a 
sudden impulse,, or been impelled by any power, or force, and af- 
terwards left to proceed by itself; the denomination being derived 
from a Latin word, the meaning of which is, to throw, to hurl. 
Thus, the bullets which are thrown out of ordnance, stones thrown 
by the hand, or by a sling, or by any other projecting instrument, 
&c. are called Projectiles. 

2. Very dense bodies moving with small velocities describe the 
parabolic track so nearly, that any deviation is scarcely discover- 
able ; but with very considerable velocities the resistance of the 
air will cause the "body projected to describe a path altogether 
different from a parabola, which will not appear surprising, 
when it is known that the resistance of the air to a cannon-ball of 
two pounds weight, with the velocity of 2000 feet per second, is 
more than equivalent to sixty times the weight of the ball. 

Fig. 9. 
Ulus 1. The horizontal range, 
Fig. 9, AB, of a body project- 
ed at an elevation of 45° is 
greater than AC or AD, the 
ranges of bodies projected with 
the same velocity at a greater 
or less elevation. If the pa- 
rallel lines EF, GH, be always 
as the squares of AE, AG, the 
curve AFH will be a parabola j 1 ^- 
and such is the path of a pro--^*- 
jected stone or cannon-ball. 




OF THE MECHANICAL POWERS. 

LXV. The Mechanical Powers are simple 
engines, founded on the principles of the laws 
of motion, which enable men to raise heavy- 
weights, move heavy bodies, and overcome re- 
sistance. 

Obs. 1. The principal moving powers are — first, the strength of 
animals, chiefly that of men and horses ; secondly, the force of 
running waters and of winds ; thirdly, the force of steam ; fourthly, 
the force of springs ; fifthly, the weight of heavy bodies. 

137. What is a Projectile ? 13S. Do bodies which are projected with 
different degrees of velocity, describe the same kind of curve exactly; 
and why not ] 139. Illustrate the difference by a diagram. 140. What 
resistance does the air oppose to a cannon-bail of two pounds weight 
moving at the rate of 2000 feet per second 1 141. What are the Mechan-, 
teal Poicers ? 142, What are the principal moving powers 1 



52 MECHANICS. 

2. The simple weight, as applied to clocks, jacks, and other 
machines, is the power which can be most easily applied as a first 
mover, and its action is almost uniform. As this power requires 
to be renewed after a certain period, it is mostly used for slow 
movements. 

3. The spring is a useful moving power, but like the weight, it 
requires to be wound up after a certain time, whence it is also 
chiefly used for slow movements. 

LXVI. Three circumstances are to be con- 
sidered in treating of mechanical contrivances ; 

1 . The weight to be raised, or the resistance to be overcome. 

2. Thepower by which it is to be raised ; and, 

3. The instruments employed. 

LXVII. The number and variety of machines 
or mechanical powers, are almost infinite ; but 
they all consist of certain parts or simple 
mechanism, variously combined and connected 
with each other. The number of those simple 
mechanical powers is usually reckoned by philo- 
sophers at no more than six ; the Lever ; the 
Wheel and Axle; the moveable Pulley, or Sys- 
tem of Pulleys /the Inclined Plane ; the Wedge j 
and the Screiv. 

Obs. 1. The active application of the doctrine of motion con- 
sists in the construction of machines for the purpose of overcoming 
resistance, or of moving bodies ; and the object of such machines 
is to increase the effect of a given power, so that the momentum 
of the power may exceed that of the resistance. 

2. Suppose the power to be 100 pounds, and the weight 1000 
pounds, powers to move the weight must be made to move with 
above ten times the velocity of the weight; and this is effected by 
means of the mechanical powers. 

Thus, if a man should be desirous of moving a stone of 2000 

Eounds weight, from a certain place, for which purpose he finds 
is strength inadequate, he would use a long bar, which being ap- 

143. What is the power most easily applied, and the action of which is 
at .the same time most uniform 1 144. In treating of mechanical con- 
trivances, how many circumstances are to be considered in order to 
understand the power of a machine 1 145. What is the first ? the 
second? the third 1 146. What is the number of simple mechanical 
powers ; and what are their names 7 147. What is the object of all 
mechanical contrivances? 



OF THE LEVER. 53 

plied in a certain manner, actually enables him to move the stone. 
Thus also a person wishing to convey some heavy article to the 
top of his house, has recourse to a set of pulleys with a rope, &c. 
and by that means easily accomplishes his object. 

3. If a man can raise "by a single fixed pulley, a beam to the top 
of a house in two minutes, he will be able to raise six such beams 
in twelve minutes ; but with a tackle having three lower pulleys, 
he will raise six beams with the same ease at once ; but he will be 
six times as long about it, that is, twelve minutes, because his 
hand will have six times as much space to pass over. 

4. Cannon-balls do much more mischief, than the battering 
rams of ancient times ; suppose the weight of a ram to be 20.000 
pounds, and to move at the rate of one foot in a second ; and the 
weight of a cannon-ball to be 24 pounds, and to move at the rate 
of 1000 feet in a second ; then the momentum or moving force of 
the ram, will be 20.000 X 1 = 20,000, and that of the cannon-ball 
will be 24 X 1,000 = 24,000; of course the effect of the latter 
will be one-fifth greater than that of the former. Thus has a 
small body a greater momentum than a large one, provided the. 
velocity of the small one be made to compensate for the greater 
quantity of matter in the other. 

LXVIII. The power of a machine is calcu- 
lated when it is in a state of equilibrium, that is, 
when the power just balances the resistance op- 
posed, and the momentum of each is equal. 

OF THE LEVER. 

LXIX. A Lever is a bar of wood, or metal, 
or other solid substance, one part of which rests 
against a steady prop called a Fulcrum, which 
supports the lever, and about which, as a centre 
of motion, the lever is moveable. 

Obs. Levers of continuous matter as wood or metal, are in 
single ratio ; but levers of elastic media which diffuse the force 
are in duplicate ratio. — Phillips. 

LXX. The possible different situations of the 
weight, the fulcrum, and the power, are not more 
than three ; hence arise three kinds of levers^ 

148. In what way can a smaller body have a greater momentum than 
a larger body 7 149. What is the Lever 1 150. What is the Fulcrum % 



54 MECHANICS. 

distinguished according to the different positions 
of the fulcrum and the moving power with re- 
spect to each other. 

Obs. In all kinds of continuous levers, the power, is to the re- 
sistance, as the distance of the resistance from the fulcrum is to 
that of the power from the fulcrum. 

LXXI. A lever of the first kind, is one having 
the fulcrum placed between the weight and the 
moving power ; as in Fig. 10. 

Fig. 10. 

Exam. If it be required to raise 
the stone a, which weighs 1000 
pounds, by the strength of a man 
equal to 100 pounds weight; a lever 
I A cc, which rests on the prop b, is placed 
with one end under the stone, and the 
man presses it down at the other end 
c. As the man's strength is only 
equal to the tenth part of the weight 
of the stone, the arm of the lever 6c, must be ten times as long as 
the arm be, in order that the power and weight may balance each 
other. 

Fig. 13. 




Fig. 11. 



Fit?. 12. 




lllus. 1. A balance is a lever of the first kind, with equal arms ; 
see Fig. 11. 

2. A steel yard, Fig. 12, is also the first kind of lever with a 
moveable weight. 

151. How many kinds of levers are generally described ; and how are 
they distinguished? 152. What is said of lovers of continuous matter, 
and Leversof elastic media? 153. What is the first kind of lever, and 
what are its uses? 154. On what part of the lever is the resistance 
placed ; and where is the power applied? 155. Draw a diagram, point- 
ing out the resistance, short arm, long arm, fulcrum, and power. 156. 
To what is the greatness of effeel produced by the lever proportional? 
157. How are we to use levers of the first kind in raising large weights 1 



OF THE LEVER. 55 

3. A poker, in the act of stirring the fire, is a lever of this kind ; 
the bar of the grate upon which it rests is the fulcrum ; the coals, 
the weight to be overcome ; and the hand is the power. 

4. An iron crow, or crow-bar, which is commonly used by- 
carpenters, blacksmiths, stone-masons, &c, is a bent lever of this 
kind, flatted at one extremity for the purpose of its being more 
easily admitted into narrow crevices. 

Obs. 1. To this kind of lever, may be referred, scissors, pincers, 
snufers, tyc. vrhich are made of two levers, acting contrary to 
each other. The fulcrum in these cases, is the pin which keeps 
them together. 

2. Levers of the first kind are chiefly used for loosening large 
stones ; or to raise great weights to small heights, in order to get 
the ropes under them. 

LXXIL The second kind of lever, is one 
-which has the fulcrum at one end, the power at 
the other, and the weight between them. 

Oos. 1. See Fig. 13, where a is the fulcrum, b the weight, and 
c the power. 

2. The advantage gained by this kind of lever, is as great, as 
the distance of the power from the fulcrum exceeds the distance 
of the weight from the same ; thus, if the hand at c be nine times 
as far from A as the point X on which the weight acts, then the 
force of one pound at c, will balance the weight b of nine pounds. 

Fig. 14. 

Illus. I. This kind of lever ex- 
plains why two men carrying a bur- 
. den, as a "cask, upon a pole, may 
bear unequal shares, according to 

i, the strength, by placing it nearer to 

5 the one than the other; seeFig. 14. 
Here the weight w, is twice nearer 
to a than b ; of course, a would bear 
twice as much weight as b. 

2. This is applicable to the case of two horses of unequal 
strength, where the beam may be so divided, that the horses shall 
draw up in proportion to their respective ability. 

3. To this kind of lever may be referred oars, rudders of ships, 

159. To what kind of mechanical power may a balance, steel-yard, 
poker, iron crow, scissors, pincers, sn uffers, ^che referred 1 160. How 
may the scissors be explained on the principle of the lever? 161. How 
is the second kind of lever designated 1 162. -Where is the fulcrum, 
power. &c. applied ? 163. To what is the advantage gained in the use of 
the second kind of lever proportional 1 164. What are the uses of the 
second kind of levers 1 165. What are the most common examples of 
levers of the second kind ? 





J4. 



56 MECHANICS. 

doors turning on hinges, and cutting knives which are fixed at 
one end. 

LXXIII. A lever of the third kind has the 
prop at one end, the weight at the other, and the 
power applied between them. Here the power 
must exceed the weight in the same proportion, 
as the distance of the weight from the prop ex- 
ceeds the distance of the power. 

Illus. Let f, Fig. 15, be the prop or 
fulcrum, p the power, and w the weight ; 
if the distance pf be only three inches, 
t and wf be twelve, then for the hand r> 
?/*to balance the weight of 20 pounds will 
require a force of four times 20, or 80 
pounds, because the weight is at four 
times the distance from the fulcrum that the power is. 

Exp. 1 . A ladder, which is to be raised by the strength of a 
man's arms, represents a lever of this kind, where the fulcrum is 
that end which is fixed against the wall, or upon which an othei 
man stands ; the weight may be considered as at the top part of 
the ladder, and the power is the strength applied to the rearing 
of it. 

2. The wheels in clock and watch work, may be reckoned levers 
of this kind, because the power that moves them acts near the 
centre of motion, by a pinion, and the resistance it has to over- 
come, acts against the teeth at the circumference. 

3. The bones of a human arm, and the greatest number of the 
moveable bones of animals, are levers of the third kind. 

rig: 16. 

To take the arm, Fig. 16, as 
an instance, — (/the elbow, is the 
centre of motion, the power is 
the motion inserted at c, about 
one-tenth part as far below the 
elbow as the hand is, and a is the 
weight to be raised ; the mnscles 
must accordingly exert a power 
equal to one hundred pounds to raise a weight of ten pounds. 

166. Id what manner may the opening and shutting of a common 
door, and the rudder of a ship, be explained as involving the principle 
of the second kind of levers 1 167. Describe the third kind of levers'? 
16«. Where is the prop or fulcrum? Where the weight? Where the 
power ? 160. What is an instance of its use ? 170. Give a natural illus- 
tration of this kind of levers? 171. How does the raising of a weight 
by the hand represent the third kind of levers? 




OF THE WHEEL AND AXLE. 57 

Corol. Hence, in natural levers, the power is disadvantageous^ 
situated, in consequence of its being so near the centre of motion, 
but the loss of power is compensated by the beauty and compact- 
ness of the limb. 

LXXIV. A hammer-lever differs only in its 
form from a lever of the first kind. 

Fig. 17. 

HI us. Let acb, Fig. 17, represent a lever of this 
kind, bended at c, which is the fulcrum, p is the 
power acting upon the longer arm a c, by means of 
the cord a d going over the pully d ; and the weight 
w acts upon the shorter arm c b, of the lever. As 
a c is five times as long as c 6, a weight of five 
pounds at p. will balance 25 pounds at w. 

Exp. If the shaft of a hammer is six times as 
long as the iron part that draws the nail, the lower 
-j"£part c resting on the board x as a fulcrum, then, by 
pulling at a, a man will draw a nail with one-sixth 
part of the power that he. must use to pull it out 
\with a pair of pincers ; in the latter case, the nail 
^ would move as fast as the hand, but in the former, 
the hand would move over six times as much space as the nail 
during the time of drawing it out of the wood. 

LXXV. The Wheel and Axle, sometimes 
called axis in 'peritrochio. consists of a cylinder, 
and a wheel fastened to the cylinder, or of a cyl- 
inder with projecting spokes, and all moveable 
round the common axis, which is supported at 
its two ends ; Fig. 18 and 19. 

LXXVI. The advantage gained is in propor- 
tion as the circumference of the wheel is greater 
than that of the axis ; or as the diameter of the 
wheels is greater than the diameter of the axis. 

172. What is the Hammer -lever ; and what are its principal uses? 
173. Of what does the second mechanical power, the Wheel and Axle, 
consist ? 174. How is the power gained by the wheel proportioned to 
the relative diameters of the wheel and axis 1 




58 



MECHANICS. 






Pis?. 18. 




Illus. If the diameter of the 
wheel, Fig. 18, or the length of 
the spoke, Fig. 19, be four feet, 
and the diameter of the axis only 
8 inches, then the power P, of one 
hundred pounds, or the strength 
of a man applied to the spokes S, 
equivalent to a hundred pounds, 
will balance a weight W of six 
V hundred pounds. 

In this case as in the le- 
ver, the power will travel 
over six times as much 
space as the weight, when 
the machine is put in mo- 
tion. 
Example 1. Cranes for 
iitiiitiiTil raising heavy weights, cap- 
stans, and windlasses, such 
as are used on board of 
ships, are engines of this sort. 

2. Sometimes the axis is turned by a winch fastened to it, which 
serves for a wheel, and the power gained is in proportion as the 
winch is larger than the axis. 

3. A capstan is a cylinder of wood, with holes in it ; into these 
bars are put to turn it round. The bars are made to act some- 
thing like the spokes ; Fig. 19. 

OF THE PULLEY. 

LXXVII. A Pulley is a small wheel, or thick 
circular piece of wood, or metal, or other solid 
matter, having its circumference grooved, in or- 
der to receive a rope, the wheel being moveable 
round a centre pin or axis, which is fixed in a 
block or frame, in a manner which need not be 
described in this place. 

UIus. 1. The small- wheel x is called a sheeve, and is so fixed 
to a block a, as to be moveable round a pin passing through the 
centre. 

175. On what mechanical principle docs the wheel and axis gain pow- 
fijrl 1/0. V\ hut is a Pulley ? 



OF THE PULLEY. 59 

LXXVIII. Pulleys are either fixed or move- 
able, 

Obs. l. The fixed pulley ^ives no mechanical advantage, but 
is used only to change the direction of a power. By it a man may 
raise a weight to any height, without moving from the place in 
which he is, as a stone to the top of a building, otherwise he must 
ascend with the weight. 

LXXIX. The moveable pulley represented 
by t, Fig. 21, is fixed to the weight, and rises 
and falls with it, and the advantage gained by 
it is as 2 to 1. 

1 . The reason of this is evident ; for in raising the weight one 
inch, foot, or yard, both sides of the rope must be shortened as 
much, that is, the hand h must move through two inches, feet, or 
yards ; which shows, as before, that the space through which the 
power moves, must always be in proportion to the advantage 
gained. 
Fig. 20. Fig. 21. 

2. When the upper jlred block, z, Fig. 
Tr= 21, contains two pullies, which only turn 
^ on their axis, and the lower moveable 
block x contains also two, which turn and 
rise with the weight W, the advantage 
gained is as four to one. For each pulley 
j?m the lower block will be acted upon by 
an equal part of the weight, and since in 
, each pulley that moves with the weight a 
^double increase of power is gained, there- 
fore the advantage gained, is as four to 
one. 

LXXX. In general the advantage gained by 
pulleys is found by multiplying the number of 
moveable pulleys by 2. 

Obs. 1. A weight W of 72 lbs. may be balanced by four movea- 
ble puUeys, by a power of nine pounds, because, 72 divided by 8 
gives 9 ; but in this case the power, when put in motion, will 




177. What is the use of a fixed pulley? 17S. How must the power 
compare with the weight in order to move it, by the use of the fixed 
pulley ? 179. What proportion of advantage is gained by the moveable 
pulley ] l=:o. What is the reason that a weight can be raised more easily 
with a pulley than without one? 151. What is the rule for estimating 
the advantage gained by pulleys? 



60 



MECHANICS. 



pass over 8 times as much space as the weight j that is, to raise 
the weight one foot, the hand must move over eight feet. 
2. A pair of blocks with a rope is called a Tackle. 



OF THE INCLINED PLANE. 

LXXXI. The inclined plane is merely a plane 
surface inclined to the horizon, and is used to 
move weights from one level to an other.— See 



Obs. It is often made by placing 

boards, or earth, in a sloping direction," 

a and is of great importance in rolling up 

^ heavy bodies, as casks, wheel-barrows 




heavily loaded, &c. 



LXXXII. The force with which a body de- 
scends upon an inclined plane, is to the force 
with which it would descend perpendicularly,-, 
as the height of the plane is to its length. Hence 
the mechanical advantage gained by an inclined 
plane is as much as the length of the plane ex- 
ceeds its height. 

Fig. 23. Fig. 24. 



ii 




A 



IUus. If the plane ab, Fig. 23, be 
parallel to the horizon, the cylinder c 
will rest on any part of it wherever 
it is laid. But if the plane be placed 
perpendicularly, as a b, Fig. 24, the 
| cylinder will descend with its whole 
weight, and will require a power 
equal to its weight, to keep it from 
descending. Or, if the plane be in- 
clined to the horizon as a d, Fig. 
22, and three times the length of the 
perpendicular b d, the cylinder e will be supported by a power 
equal to a third part of its weight. And if the plane be 20 feet 
lon.g, and the perpendicular height be 4 feet, or one-fifth,) then a 
weight of five hundred pounds would be balanced upon it by an 

182. What name is used to denote a pair of blocks with a rope 1 
183. What is an Inclined Plane? 184. Of what common or evefy day 
use is the inclined plane? 



B 



OF THE SCREW. 61 

other of one hundred pounds, because the plane is five times the 
length of the perpendicular height to which the weight is to be 
raised. To the inclined plane may be reduced hatchets, chissels 
and other edge tools, which are sloped only on one side. 

OF THE WEDGE. 

LXXXIII. The Wedge may be considered as 
two equally inclined planes united at their bases. 
The advantage gained is in proportion as the 
length of the two sides of the wedge is greater 
than the back, or as the length of one side is 
greater than half the back. 

Fig. 25. 

~\7" ' _ III us. The wedge ab c d x, Fig. 25, may be 
1 - divided into two inclined planes, a v ex, and b v 
^cbd z x, which may be used separately, and will 
gain advantage as such ; therefore, when united 
at z x, the advantage gained will be in the same 
proportion, as when they were used in different 
parts. 

Obs. When the wood cleaves at a distance 
before the wedge, the advantage then gained is in 
proportion as one side of the cleft is greater than half the length 
of the back. 

LXXXIV. The wedge is a very important 
mechanical power, used to split rocks, timber, 
&e. which could not be effected by any other 
mechanical power. 

Obs. All cutting instruments, and some sorts of chissels cham- 
fered on both sides, are to be refered to the principle of the wedge. 

OF THE SCREW. 

LXXXV. The Screw is a cylinder, which has 
either a prominent part, or a hollow line, passing 

185. How is the velocity with which a body descends on an inclined 
plane, proportioned to the velocity with which it descends perpendicu- 
larly? ~IS7. Illustrate this by means of a diagram. 136. How much is 
the resistance of the weight diminished by the use of an inclined plane % 
188. Of what is the Wedge composed ; and on what principle does it act? 
190. In what does the resistance of the wedge consist 1 1S9. On what 
mechanical principles are cutting instruments designed ? 191. What is 
the Screw? 

6 




62 MECHANICS. 

round it in a spiral form, so inserted in one of 
the opposite kind, that it may be raised or de- 
pressed at pleasure, with the weight upon its 
upper surface, or suspended beneath its lower 
surface. 

Obs. The screw is likewise considered as a species of inclined 
plane, used with a lever or winch to assist in turning it ; and then 
it becomes a compound engine of great force, either in pressing 
bodies closer together, or in raising great weights. In fact, a 
screw is simply an inclined plane coiled round a cylinder, and the 
nut or perforated body wliicn moves up or down a screw, moves 
up or down an inclined plane in a circular instead of a rectilinear 
direction. And it may be conceived to be made, by cutting a 
piece of paper into the form of an inclined plane, and then wrap- 
ping it round a cylinder ; the edge of the paper will form a spiral 
line round the cylinder, which line will answer to the thread of 
the screw. 

LXXXVI. The advantage gained by this 
kind of mechanical power, is in proportion as the 
circumference of the circle made by the lever or 
winch is 
threads of the screw 



greater than the distance between the 



Fig, 



Fig. 27. 




Mils. It is evident that the winch or 
lever will turn the cylinder once round, 
whilst the weight, or the resistance, 
can be moved from one spiral winding 
to an other, as from x to 2r, Fig. 26. If 
the distance of the spirals z x, is half 
an inch, and the lever a, three feet, of 
3G inches long, then the circle described 
by the lever, will be about 228 inches, or 456 half inches, conse- 
quently a force at the end of the lever, equal to only one pound, 
would balance a resistance at the thread of 456 pounds. Hence 
it appears, that the longer the winch or lever, and the nearer the 
spirals, the more advantage is gained. But in the screw there 
is great loss of power ; for a screw may be moved upward or 

192. What is the construction of the screw and nut? 193. What is the 
rule by which the forr<> of a screw and lever is estimated ? Or what is 
the advantage ff.-iinrd ? 194. How can the power of the screw be in- 
creased ? 196. What is the principle of the screw 1 196. Illustrate this 
principle by an explanation of the figure. 



OF THE SCREW. 63 

downward in a fixed nut, as in Fig. 26, or the nut may move on a 
screw, as in Fig. 27. 

Obs. 1. Almost all kinds of presses, common cork-screws, gim- 
blets, augurs, &c. act upon the principle of this kind of mechani- 
cal power. 

When a screw turns in a wheel, it is called an Endless Screw. 
By way of distinction, the cylinder with the spiral protuberance, 
is sometimes called the Male Screw ; and the nut, with its perfo- 
ration shaped like a hollow screw, is called the Female Screw. 

2. A little reflection on the preceding explanation of the nature 
and properties of the mechanical powers, will sufficiently prove 
that, in a strict philosophical point of view, the real and original 
mechanical powers are not more than two in number ; being all 
referable to the lever and inclined plane ; so that all the others are 
only species of these two ; the wheel and axle, and the pulley be- 
ing species of the lever; and the wedge with the screw being 
species of inclined plane. And likewise, the various philosophi- 
cal writers do not agree with respect to the number of the sim- 
ple mechanical powers ; some reckoning two, others five, and 
some have enumerated seven, &c. It is, however, immaterial 
whether those powers are considered all primitive and distinct 
from each other, or not ; for the theory always remains true and 
the same. 

AH the instruments or machines which connect an active force 
with a certain effect, however complicated they may be, will, 
upon examination, be found to consist of the already described 
mechanical powers. Those component simple mechanisms are 
frequently varied in shape ; their connexion is infinitely diversi- 
fied ; but their nature and their properties remain invariably the 
same. 

3. Beginners in this branch of Natural Philosophy frequently 
imagine that by means of the mechanical powers, a real increase 
of power is obtained ; which idea is very far from being correct. 
In considering the advantage gained by machines, it may be ob- 
served, geuerally, that whatever is gained in power is gained by 
an increase of motion. Thus, with the screw, to raise a weight as 
much as the thickness of one thread, the handle or winch must 
perform an entire revolution. 

The amount of power gained by machinery may indeed be im- 
mense, but it is gained either by a proportional increase of velo- 
city in the moving power, or a proportional sacrifice of time in 
performing the operation. 

By the use of machinery, man seems to have acquired a kind of 
empire over nature, notwithstanding all the imperfections of ma- 

197. What instruments, machines, or mechanical contrivances, act 
upon the principle of this kind of mechanical power 1 19S. What is an 
Endless Screw ? 199. What is a Male and Female Screw ? 200. What 
is the real and original number of the simple mechanical powers ? 
201. In what consists the advantage of any of the mechanical powers, if 
there is as much loss of time as gain of power 1 



64 OF FRICTION. 

chines, and the various impediments by which their effects are 
counteracted. While the winds, the streams of water, or the ex- 
pansive force of steam perform our labour, they only require us to 
superintend and regulate their operations. 



OF FRICTION. 

LXXXVII. The power, or acting force, is so 
far from being increased by any machine, that a 
certain part of its effect is always lost in over- 
coming the resistance of mediums, the friction, 
or other unavoidable imperfections of machines. 
And this loss in some compound engines is very 
considerable. 

Obs. 1. In the operation of any machine, there is always some 
icciglit to be moved, or resistance to be overcome. To effect this, 
some force or power must be applied, which must always be superior 
to the resistance,; otherwise the machine can not be put in motion. 

2 It has been ascertained, that in the application of all the me- 
chanical powers, one-third at least must be allowed to overcome the 
friction of the surfaces, and the various other obstacles to which 
'all machines are liable. 

3. If 60 pounds are required to balance any weight with a me- 
chanical power, 80 pounds will be wanted, owing to friction, to 
put the machine in motion. 

4. Friction is the resistance a moving body meets with from 
the surface over which it passes ; it is of two kinds, the rubbing 
by friction, and the friction by contact. The former is represented 
by a locked wagon-wheel going down a hill, the second by the 
wheel touching the ground in its usual motion. The force of fric- 
tion varies in proportion to the different surfaces in contact ; thus, 
a marble passing on a smooth pavement suffers less from friction 
than it would from gravel, and it would be impeded in its motion 
still less if it were driven over ice. But the hardest and most 
polished bodies are not free from inequalities that retard their 
motion when they act upon each other. The smallest impediment 
from friction is, when finely polished iron is made to rub on bell- 
metal, but even these are said to lose about one-eighth of their 
moving power. The friction between rolling bodies is much less 

202. What is said of the utility and importance of machinery'? 203, 

What diminishes the forre of all machinery? 204. What are we to un- 

m1 by Fr'ntion, in machinery ? 205. In what proportion does the 

friction of machinery destroy the effect of a given power? 206, Wha?, 

ore the kinds of friction 7 

3* 



HYDRODYNAMICS. 65 

than in those that drag ; hence, in certain kinds of wheel "work, 
the axle is made to move on small wheels, or rollers in the inner 
circumference of the nave. These are denominated friction roll- 
ers, and are so placed together in a box. and fastened in the nave, 
that the axle of the carriage may rest upon them, and they turn 
round their own centres as the wheel continues its motion. "Fric- 
tion rollers do not answer in very heavy machines, as the pressure 
is apt to wear the naves into notches ; but in light and rapid mo- 
tions they are extremely useful. Larger metal balls, on the same 
principle", are made use" of in moving immense blocks of stone. 

5. Of the simple mechanical powers, the lever is the least sub- 
ject to friction ; but in the screw, and common pulley, especially 
those of small size, the friction is very great. 

6. After a great variety of experiments made with the utmost 
care and attention, respecting friction, Mr. Vince has deduced the 
following conclusions, which may be considered as established 
facts. 

I. That friction is a uniformly retarding force in hard bodies, 
not subject to alteration by the velocity, except when the body is 
covered with woollen cloth, &c. and in that case the friction in- 
creases a little with the velocity. 

II. Friction increases in a less ratio than the weight of the body, 
being different in different bodies. It is not yet sufficiently known 
for any one body, what proportion the increase of friction bears to 
the increase of weight. 

III. The smallest surface has the least friction ; the weight 
being the same. But the ratio of the friction to the surface is not 
accurately known. 

HYDRODYNAMICS. 
OR THE LAWS OF FLUIDS. 

LXXXVIII. That branch of Natural Philoso- 
phy which relates to the phenomena of water 
and other fluids, whether at rest or in motion, 
is usually called Hydrodynamics. This science 
is commonly divided into two branches ; Hydro- 
statics and Hydraulics. 

LXXXIX. The science of Hydrostatics re- 
lates to the pressure, gravity, equilibrium, and 

237. What is the use of Friction Rollers! 20S. What are the conclu- 
sions of Mr. Vince. on the subject of friction? 209; What is to be under- 
stood by the science of Hydrodynamics! 210. What are the two 
branches into which it is commonly divided 1 2 LI. Of what does the 
science of Hydrostatics treat 1 

6* 



66 HYDROSTATICS. 

cohesion of fluids in general, and of the method 
of weighing solids in them. And its mechanical 
practice, Hydraulics, relates particularly to the 
motion of water through pipes, &c, the resistance 
which fluids oppose to moving bodies, and the 
construction of the various machines or engines 
in which fluids are principally concerned. 

XC. A Fluid is a body, the parts of which 
yield to any force impressed upon them, and are 
easily moved out of their places, or amongst 
themselves. 

Fluids are of two kinds — those which are elastic and compressi- 
ble, as atmospheric air, steam, and the different gases ; and those 
which are non-elastic and incompressible, as water, oil, mercury, 
&c* It is with the latter class, or rather with water, that we are 
now particularly concerned. 

Obs. Water, and liquids in general, appear to be scarcely sus- 
ceptible of being compressed into a smaller space than that which 
they naturally occupy. This is supposed to be owing to the ex- 
treme minuteness of their particles, which, rather than submit to 
compression, force their way through the substance which confines 
them. Water is compressible, however, by very strong pressure — 
a fact which was long disputed, but which has been recently esta- 
blished by Mr. Perkins, who found that the pressure of 2000 at- 
mospheres occasions a diminution of one-twelfth of its bulk. 

2. Heat, or motion, is supposed to be the cause of fluidity ; for 
example, ice, without heat, is a solid — with heat, it becomes a fluid, 
in water — and with more heat, an elastic fluid, in steam. In the 
first state, the atoms are fixed in crystals — in the second, are 
thrown into intestine motion — and in the third state, are forced 
asunder with an amazing expansive force. 

3. Philosophers have usually assumed, that the particles of 
fluids are round and smooth, since they are so easily moved among 
themselves. This supposition will account for some circumstances 

212. What is the difference between Hydrostatics and Hydraulics ? 
213. What is a. Fluid? 214. How are fluids distinguished? 215. Iswa- 
ter absolutely incompressible? 216. What does the experiment of Mr. 
Perkins prove? 217. What is the cause of fluidity? 

• Waler, oil, spirit of wine, and other such fluids, are said to be non- 
elastic or incompressible, not because they are absolutely so, but because 
their compressibility under existing circumstances, is so very small, as 
to make no sensible difference in our calculations relative to the pres- 
sure, movements, and other properties of those fluids. 



HYDROSTATICS. 67 

belonging to them. If the particles are round, there must be va- 
cant spaces between them, in the same manner as there are vacui- 
ties between cannon-balls that are piled together; between these 
balls, smaller shot may be placed, and between these, others still 
smaller, or gravel, or sand, may be diffused. In a similar manner, 
a certain quantity of particles of sugar can be taken up in water 
without increasing the bulk ; and when the water has dissolved 
the sugar, salt may be dissolved in it, and yet the bulk remain the 
same ; and admitting that the particles ol water are round, this 
is easily accounted for: 

4. Others have supposed, that the cause of fluidity is the mere 
want of cohesion of the particles of water, oil, &c. and from this 
imperfect cohesion, fluids, in small quantities and under peculiar 
circumstances, arrange themselves in a spherical manner, and 
form drops. 

XCI. Fluids are subject to the same laws of 
gravity with solids ; but their want of cohesion 
occasions some peculiarities. The parts of a 
solid are so connected as to form a whole, and 
their weight is concentrated in a single point, 
called the centre of gravity ; but the atoms of a 
fluid gravitate independently of each other. 

XCIL Fluids press not only like solids, per- 
pendicularly downwards, but also upwards, side- 
ways, and in every direction. 

Exp. 1. Take a glass tube open at both ends, put a cork in one 
end, and immerse the other in water. The fluid will not rise far 
in the tube ; but the moment the cork is taken out, it will rise to 
a level with the surrounding water ; which proves the pressure 
upwards. 

Obs. The upward pressure of fluids must not be understood to 
be a positive property ; for water, it is evident, never presses in 
this direction except when it is forced to do so by the downward 
pressure of some higher part of the same body of water. 

XCIII. A fluid which is kept in an open ves- 
sel, or in several vessels communicating with 

21?. What is inferred from the slight cohesion of the particles of 
fluids, and the facility with which they slide over each other ? 219. How 
do fluids gravitate, in comparison with solids 1 220. In what direction 
do fluids press 1 221. What simple experiment proves the upward 
pressure of a fluid 1 222. What is said of liuids kept in open vessels 1 




68 HYDROSTATICS. 

each other and open at top, will assume a flat 
surface parallel to the horizon, and will remain 
at rest or rise to a common level. 

Fig. 28. 

Exp. 1, If a vessel. Fig. 28, consists of pipes 
variously inclined, communicating with each 
other at b, and open at the top, water poured 
into any one of them will rise to the same level 
zf, in all. 

2. Let water spout upwards through a pipe having a small ori- 
fice inserted into the bottom of a deep vessel ; it will rise nearly 
to the height of the upper surface of the water in the vessel. The 
resistance of the air, and of the falling drops, prevents it from 
rising perfectly to the level. 

XCIV. The pressure of the same fluid is in 
proportion to the perpendicular height, and is 
exerted in every direction ; therefore, at the same 
depth, all the parts or particles of the fluid press 
against each other with equal force in every di- 
rection. 

Exp. I. If a bladder full of air be immersed in water, then the 
perpendicular pressure is .manifest, for the deeper the bladder is 
immersed, the more will its bulk be contracted. 

2. An empty bottle being corked, and by means of a weight, let 
down a certain depth into the sea, it will be broken, or the cork 
will be driven into it by the perpendicular pressure. But a bottle 
filled with water, wine, &c. may be let down to any depth, with- 
out damage, because in this case the internal pressure is equal to 
the external. 

3, It is evident, that the quantities of water in the different 
pipes, Fig. 28, whatever be their size, press equally against each 
other, for "if the water be suddenly taken out of the pipes e, 2r, or,/*, 
the surface of the water will instantly descend to a lower level in 
all the other pipes. 

Obs. At the depth of 32 feet below the surface of the sea, a diver 
has been calculated to be pressed with a weight equal to about 
28,000 avoirdupois pounds; yet as that pressure is distributed all 
over his body, and the human body consists mostly of non-elastic 

223. What relation is observable between the height of a fluid and its 
perpendicular pressure? 224. What simple illustrations may be given 
of these principles 1 225. What degree of pressure does a person sustain 
at the depth of thirty -two feet below the surface of the sea 1 



HYDROSTATICS. W 

fluids or of solids, he does not feel any remarkable inconvenience 
from such a pressure. 

XCV. The horizontal bottom of a vessel sus- 
tains the pressure of a column of the fluid, the 
base of which is the bottom of the vessel, and 
the perpendicular height equal to the depth ol 
the fluid ; or, in all cases, the pressure of fluids is 
proportional to the base and the perpendicular 
height of the fluid, whatever be the form of the 
vessel or the quantity of the fluid. 

Fig. 29. Fig. 30. Fig. 31. 




Exp. 1. In the vessel a b, Fig. 29, the bottom c 6 does not sus- 
tain a pressure equal to the quantity of the whole fluid, but only 
of a column, whose base is c b, and height b a. 

■2. In the vessel f g, Fig. 30, the bottom g sustains a pressure 
equal to what it would be if the vessel was as wide at the top as 
the bottom. 

3. Let a gun barrel, four feet in length, be fixed into a box, four 
feet square and one inch in height, and then the box and barrel 
filled with water ; the pressure of the water upon the bottom of the 
vessel, or the downward pressure, will be the same as if the box 
were of a cubical form, four feet in height and full of water. 

4. Let two tubes of different forms be successively applied to 
the same moveable circular base, suspended by a wire passing 
from the centre of the base through the tubes, to the beam of a 
balance ; when the different tubes are filled to the same height, it 
will require the same weight at the opposite end of the balance to 
keep the base from sinking. 

XCVI. Hence the origin of what has been de- 

226. How is the pressure of a fluid on the bottom of a vessel esti- 
mated 1 227* Illustrate these principles by means of the diagrams. 



70 HYDROSTATICS. 

nominated the Hydrostafical Paradox, which is, 
that any quantity of fluid, however small, may 
be made to counterpoise any quantity, however 
large, 

Exp. If to the wide vessel a b, Fig. 31, we attach a tube c d, 
and pour water into either of them, it will remain at the same 
height in both. Of course, the small quantity in c d balances the 
large quantity in a b. But this is only a paradox in term, because 
the action of the fluid is downward not upward. 

XCVII. The pressure of a uniform fluid upon 
any given part of the bottom or sides of a vessel, 
is equal to the w;eight of a column of that fluid, 
having a base equal to that part of the bottom or 
side, and an altitude equal to the perpendicular 
height of the fluid. 

Obs. 1. Hence may be calculated the pressure upon dams, pens, 
cisterns, pipes, aqueducts, dikes, floodgates, &c. and consequently 
the strength required for such contrivances. 

2. The pressure of fluids differs from their gravity or weight in 
this ; the weight is according to the quantity, but the pressure is 
according to trie perpendicular height. 

3. From this property also, we ascertain the principle of spout- 
ing fluids. If a hole is bored in the side of an upright pipe filled 
with water, the fluid will spout out, which shows the lateral pres- 
sure, and this pressure is so much greater, in proportion as the 
hole is farther removed from the surface ; that is, a hole three feet 
below the surface of a vessel of water, will throw out, in the same 
time, much more water than one, only a single foot below. 

The momentum or force with which any fluid runs out of a 
given orifice in the bottom or side of a vessel, is proportional to 
the perpendicular depth of the orifice below the surface of the 
fluid; and is also proportional to the square of its velocity, or to 
the square of the quantity of matter. 

XCVITI. A small quantity of fluid may be 
made to press with a force sufficient to raise a 
great weight. This and the upper pressure of 
fluids may be readily and pleasingly illustrated 

228. What is the ITtjdrostatical Paradox 1 220. What is its princi- 
ple? 230. In what does the pressure of a fluid differ from its gravity'} 
W . What is said of the momentum of a fluid issuing from an orifice ? ' 



HYDROSTATICS. fl 

by means of an instrument called the hydrosta- 
tical bellows. 

E:cp. 1. This machine consists of two oval boards, 16 inches 
in length, and about 14 in width, covered with leather, to rise and 
fall like the common bellows, but without valves. A pipe three 
feet long is fixed to the top board ; let a little water run into the 
bellows to separate the boards, then weights to the amount of two 
or three hundred pounds may be put on the upper board ; after 
which if the pipe be supplied with water, it will, by its upper 
pressure, sustain and lift the weights. 

2. By means of this property of fluids, the strongest vessels, as 
a tight iron-bound cask, for instance, may be burst with a very 
small quantity of water more than is necessary to fill it; for by 
filling with the fluid a tube of considerable length, securely fixed 
in the cask, the pressure of the. small quantity will show itself in 
its singularly powerful effects upon the cask, provided the tube 
be of sufficient length. 

3. Upon the principle of the upward pressure, lead or other 
metal may be made to swim in water. Into a vessel of water 
plunge a glass tube, open throughout ; but by a string hold a flat 
piece of lead \ of an inch thick, fast to the bottom of the tube, to 
prevent the water from getting in between the lead and the glass. 
In this situation, if the tube is immersed in the vessel of water to 
about three inches depth, the string may be let go, but the lead will 
not fall ; it will be kept adhering to it by the upward pressure be- 
low it. The lead being about eleven times heavier than wa- 
ter, and the three inches being eleven times the thickness of the 
lead, is the reason that depth is fixed on. Had iron been used, 
the depth must have been less than two. inches, because iron is 
seven or eight times heavier than water ; and if the plate had been 
of gold, the depth to which it must have been plunged, would have 
been nearly five inches, because gold is eighteen or nineteen- times 
heavier than water. 



OF SPECIFIC GRAVITY. 

XCIX. By the Specific Gravities of bodies, 
is meant the relative weights which equal bulks 
of different bodies have in regard to each other. 
Or the Specific Gravity of a body is its weight, 

232. What is the Hydrostatical Bellows ; and upon what principle 
does it act 1 233. In what manner may the upward pressure of fluids 
be illustrated, as occasioned bv the downward pressure'? 234. How may- 
lead be made to swim upon water 1 23o. What is meant by the SuecnTe 
Gravity of a body 1 



72 HYDROSTATICS. 

compared with that of an other body of the same 
magnitude. 

Obs. 1. Thus a cubic foot of cork is not of equal weight with a 
cubic foot of water, or marble, or lead ; but the water is four times 
heavier than the cork, the marble 11 times, and the lead 45 times ; 
or, in other words, a cubic foot of lead would weigh as much as 
45 of cork, &c. &c. 

2. The terms Absolute Gravity and Specific Gravity very fre- 
quently occur in Physics. The first is what we express in com- 
mon life by the word weight, and signifies the whole of the power, 
with which a body presses downward to the Earth. Every par- 
ticle in every substance is heavy ; that is, it has a tendency to fall 
toward the Earth, or is attracted by the Earth. Now, the greater 
the number of particles a substance has, the more powerful will 
be its tendency toward the Earth ; and we express the degree of 
this tendency by certain quantities, on which we have fixed as 
standards, by ounces, pounds, &c. Thus w T e say, rightly, this 
stone weighs sixteen times as much as this ounge. It is very 
common, however, to say, that stone is as heavy again as this ; 
that pound weight is sixteen times as heavy as this ounce : but 
this, in a physical sense, is improper; the two stones, if they' be 
of the same kind, are equally heavy, bulk for bulk. Their heavi- 
ness or gravity is the same, but their weight is different. 

O. It is usual to compare the weight of bodies 
with that of water, as it is by weighing them in 
water that their various specific gravities are 
most conveniently ascertained. 

Obs. The method of ascertaining the specific gravities of bodies 
was discovered accidentally by Archimedes. He had been em- 
ployed by the king of Syracuse to investigate the metals of a 
golden crown, which he suspected had been adulterated by the 
workman. The philosopher laboured at the problem in vain, till, 
going one day into the bath, he perceived that the water rose in 
the bath in proportion to the bulk of his body ; he instantly saw 
that any other substance of equal size would have raised the 
water just as much, though one of equal weight and of less bulk 
could not have produced the same effect. He immediately felt 
that the solution of the king's question was within his reach, and 
he was so transported with joy, that he leaped from the bath, and 
running, naked through the streets, cried out, " Evprjica, ~Evpr)Ka » 

" I have found it out, — / have found it out /" He then got 

two masses, one of gold and one of silver, each equal in weight 

236. What is the difference between Absolute gravity and Specific gra- 
vity 1 237. By what method are the specific gravities of bodies most con- 
veniently ascertained ? 



OF SPECIFIC GRAVITY. 73 

to the crown, and having filled a vessel very accurately with 
water, he first plunged the silver mass into it. and observed the 
quantity of water that flowed over ; he then did the same with the 
gold, and found that a less quantity had passed over than before. 
Hence he inferred that, though of equal weight, the bulk of the 
silver was greater thau that of the gold, and that the quantity of 
water displaced was, in each experiment, equal to the bulk of the 
metal. He next made a like trial with the crown, and found it 
displaced more water than the gold, and less than the silver, which 
led him to conclude, that it was neither pure gold, nor pure silver. 

CI. A fluid specifically lighter than an other 
fluid will float upon its surface. 

For the lighter fluid will be less powerfully 
acted upon by the force of gravitation than the 
heavier ; whence, the heavier will take the lower 
place. 

Exp. 1. Let a small and open vessel of wine be placed within 
a large vessel of water, the wine will ascend. 

2. Let mercury, water, wine, oil, spirits of wine, be put into a 
phial in the order of their specific gravities ; they will remain 
separate. 

CIL If a body floats on the surface of a fluid 
specifically heavier than itself, it will sink into the 
fluid till it has displaced a portion of fluid equal 
in weight to the whole solid. 

Obs. A body, floating on a liquor specifically heavier than itself, 
will sink into it, till the immersed part takes up the place of so 
much fluid as is equal to it in weight. For, in that case, that part 
of the surface of the fluid upon which the body rests, is pressed 
witli the same degree of force, as it would be was the space full 
of the fluid ; that is, all the parts of the surface are pressed alike, 
and, therefore, the body, after having sunk into the fluid till it is 
in equilibrio with it, will remain at rest. 

Exp. 1. Place a cube of wood in a small jar, exactly filled with 
water; a part of its bulk will be immersed, and will displace a 
quantity of the water. Take the cube out of the water, and put 
it into a scale, with which an empty vessel in the other scale 
stands balanced. Then pour water into that vessel till the equi- 

23i Hovr and by whom was the method of ascertaining the srocifio 
gravities of bodies" first discovered ?- -239. Why will not a body specific- 
ally lighter than water, sink to any depth in that fluid ? 210. What 
simple experiments are illustrative of this doctrine ?- 



74 HYDROSTATICS. 

librium is restored, and that portion of water will exactly 411 t*p 
the jar in which the cube was placed. Consequently, the weight 
of the water displaced is exactly equal to the weight of the wood. 
2. Let a glass jar, with a weight sufficient to make it sink in 
water to about two-thirds of its length, be placed first in a large 
vessel of water, and afterwards in one which is very little wider 
than the jar, and which has in it a small quantity of water ; the 
jar will sink to the same depth in both vessels, that is, till so much 
of the vessel is under water as is equal in bulk to any quantity of 
fluid whose weight is equal to that of the whole vessel. 

CIII. A body immersed in a fluid will sink to 
the bottom, if it be heavier than its bulk of the 
fluid ; and if it be suspended in it, it will lose as 
much of what it weighed in air, as its bulk of 
the fluid weighs. 

CIV. All bodies equal in bulk, which would 
sink in fluids, lose equal weights when suspended 
therein ; and unequal bodies of equal weights 
lose in proportion to their bulks. 

Obs. This is the foundation of the whole doctrine of specific 
gravities. The fluid is a common and uniform measure of weight, 
with which the other bodies are compared and contrasted. 

CV. If the same body be successively placed 
in fluids of different specific gravities, it will dis- 
place different quantities of those fluids ; that is, 
it will sink deeper in the lighter than in the 
heavier fluid ; and if the weight of the body be 
equal to that of the same bulk of the fluid, then 
it will remain at rest in any part of that fluid, 
below the surface, and no part of the body will 
appear above the surface of the fluid. 

CVI. A solid weighs less when immersed in a 

241. What is the foundation of the doctrine of specific gravity ? 242. 
What body has been adopted as a standard of reference, in the doctrine 
of specific gravity 1 213. What is said of the same body, when success- 
ively placed in fluids of different specific gravities 1 



OF SPECIFIC GRAVITY. 



75 



fluid than in open air, by the weight of a 
quantity of the fluid equal in bulk to the solid, 

CVII. The specific gravity of a body may 
easily be determined by weighing the body first, 
in the air, and afterwards in water ; and observ- 
ing the difference. The weight which the body 
loses in water, is to its whole weight as the 
specific gravity of that fluid is to that of the 
body. 

CVIII. The instrument commonly used in 
weighing bodies in order to determine their spe- 
cific gravities, is called the Hydrostatic Balance. 
and differs but little from a common balance. It 
has a hook at the bottom of one of the scales, on 
which the different substances to be examined, 
may be suspended by a silken thread or horse- 
hair. 

Mg. 32. 

Jllus. If a "body x, Fig. 32, suspended 
under the scale, be first counterpoised 
in air by weights in the opposite scale, 
.and immersed in water, the equilibrium 
will be destroyed ; then if a weight be 
put into the scale from which the body 
nangs to restore the equilibrium, that 
weight will be equal to the weight of 
water as large as the immersed body. 

Obs. The instrument used for com- 
paring the specific gravities of liquids 
is called a Hydrometer ; and is used 
to prove or ascertain the strength of 
spirits, try the strength of wort, and ex- 
amine the saturation of brine in salt- 
works. The deeper the hydrometer 
sinks in spirits, the better they are ; in 
worts and brine the contrary. Hydro- 
meters are commonly hollow balls of glass, with a smaller ball, 

244. How much less does a solid weigh in a fluid than in the open air 1 
245. What is the practical method of determining the specific gravity of 
a body 1 2i6. What kind of instrument is commonly used for deter- 
mining the specific gravities of bodies 7 247. What is its construction 1 




76 HYDROSTATICS. 

containing quicksilver at the bottom, and a slender tube at the top. 
The tube or stem is graduated, that the depth to which it sinks 
may be known. 

CIX. The specific gravity of all bodies that 
sink in water may be found, first by weighing 
the body in air, then in water, and dividing the 
weight in air by the loss of weight in the water. 

Exam. A guinea weighs 129 grains in air; by being weighed 
in water it loses 7\ grains, which shows, that a quantity of water 
of equal bulk with the guinea, weighs 7\ grains; divide 129 by 
7j, or 7.25, and the quotient will be 17.793, which proves the 
guinea to be 17.793 times heavier than its bulk of water. 

Corol. 1. We hence easily deduce the methods of obtaining the 
specific gravities of all bodies, taking rain water as a standard, a 
cubic foot of which being uniformly found to weigh 1000 avoirdu- 
pois ounces. 

Since the weight which a body loses, in a fluid, is to its whole 
weight, as the specific gravity of the fluid is to that of the body, 
— and three terms of the proportion being given, the fourth may 
consequently be easily found; as for example ; suppose a piece of 
metal, as a guinea, to weigh in air 129 grains, and in being im- 
mersed in water lose 7\ of its weight, the proportion will be 7\ : 
129 :: 1000: the specific gravity of a guinea. By this method, 
the specific gravities of all bodies that sink in water may be 
found, and expressed in a table. 

2. Hence, if different bodies be weighed in the same fluid, their 
specific gravities will be as their whole weights directly, and as 
the weights lost inversely. 

If a body to be examined consist of small fragments, they may 
be put into a small bucket and weighed ; and then if from the 
weight of the bucket and body in the fluid, we subtract the weight 
of the bucket in the fluid, there remains the weight of the body in 
the fluid. 

3. If the same body be weighed in different fluids, the specific 
gravity of the fluids will be as the weights lost. 

The loss of weight sustained by a glass ball in water and milk 
is respectively 803 and 831 grains, therefore the specific gravity 
of water is to' that of milk as 803: 831, that is, as 1.000: 1.034. 
By the same method, the specific gravity of water, is to that of 
spirits of wine as 1.000 to 857. 

2!S. What are the construction and use of a Hydrometer 1 249. What 
is the rule for finding the specific gravity of a body after weighing it in 
air and in water? 25 >. How may the specific gravity of a fluid be as- 
ined. 



Oi?' SPECIFIC GRAVIT1. 77 

SPECIFIC GRAVITIES OF VARIOUS BODIES. 



Platina [crude] 
Platina [pure] - • 

Gold 

Standard gold - - 
Silver - - - - 
Standard Silver - - 
Mercury - - - 
Copper - - - - 
Iron - - - - 

Steel - - - - 
Cast iron - - - 
Tin ... - 

Lead - - - - - 

Zinc - - - - 
Nickel - - - - 
Bismuth - - - 
Antimony - - - 
Cobalt - - - ■ 

Arsenic - - - 

Tungsten - - - 
Manganese - - 
Tellurium - - ■ 
Uranium - - - 
Brass - - • 

Gun metal - - 

Ceruse [white lead] 
Magnet - - - 

Diamond - - ■ 

Marble - - - - 
Flint - - - - 
Green glass - - 
Lime - - - ■ 

Chalk - - - 

Potass - - - 

Magnesia - - - 
Calculus humanus 
Sulphur _ - - 

Phosphorus - - 
Pit coal - - - 

Charcoal [heaviest] 
Amber ' - - - 



16.400 

22.640 

19.300 

18.833 

10.511 

10.201 

13.530 

8.730 

7.780 

7.850 

7.434 

7.900 

11.353 

7.000 

S.240 

9.822 

6.700 

8.240 

8.310 

17.400 

6.850 

- 6.185 
6.440 

- 8.350 
8.734 

- 3.160 
4.930 

- 3.520 
2.716 

- 2.580 
2.640 

- 1.524 
1.794 

- 1.709 
0.346 

- 1.542 
1.991 

- 1.714 
1.328 

- 0.441 
1.073 



Ivory - - - 

Butter - - - 

Tallow - - - - 
Lard - - ' - 

Beeswax - - - - 
Linseed oil - - - 
Train oil - - - • 
Olive oil - - - 
Common gum. 
Gum Arabic - - 
Assafcetida - - • 
Alcohol - - - 

Brandy • - 

Rain water - - - 
Sea water - - ■ 
Cow's milk - - 
Human milk - ■ 
Ice - - - - 

Living men - ■ 

Lignum vitce - - 
Box wood [French] 
Box wood [Dutch] - 
Ebony - - 

Heart of old oak - 
Mahogany - - 
Beech tree - - - 
Apple tree - - 
Plum tree - - - 
Maple - - - 

Cherry - - - 

Quince - - - 
Pear tree - - - 
American cedar - 
Pine tree - - - 
Brazil wood - - 
Logwood - - - 
Cork - - - 

Common air - - - 
Oxygen gas - - 
Nitrogen gas - - 
Hydrogen gas 



- 1.825 
• 0.942 

- 0.942 
■ 0.957 

- 0.966 

- 0.940 

- 0.924 

- 0.915 

- 1.432 

- 1.452 

- 1.328 

- 0.829 

- 0.837 
1.000 

- 1.026 
1.032 

- 1.020 
0.908 

- 0.891 
1.327 

- 0.912 
1.323 

- 1.209 

- 1.170 

- 1.063 

- 0.852 

- 0.793 
0.785 

- 0.755 
0.715 

- 0.705 

- 0.661 

- 0.560 
0.550 

- 1.031 

- 0.913 

- 0.240 
0.001208 
0.001342 
0.000338 
0.001174 



Obs. 1. The above table shows the specific weights of the vari- 
ous substances contained in it, and the absolute weight of a cubic 
foot of each body is ascertained in avoirdupois ounces, by multi- 
plying the number opposite to it by 1000, the weight of a cubic foot 
of water, thus the weight of a cubic foot of mercury is 135S0 
ounces avoirdupois, or 8491b. 

7* 



78 HYDRAULICS. 

2. If the weight of a body bo known in avoirdupois ounces, its 
weight in Troy ounces will be found by multiplying it into .91145. 
And if the weight be gives in Troy ounces, it will be found in mul- 
tiplying into 1.0971. 

3. Mr. Robertson, late librarian to the Royal Society, investi- 
gated the specific gravity of living men, in order to know what 
quantity of wood would be sufficient to keep a man afloat in wa- 
ter, supposing that most men were specifically heavier than river 
water, but the contrary appeared to be the case from trials which 
he made upon ten different persons, whose mean specific gravity 
was, as expressed in the table, 0.891, or about one-ninth less 
than common water. 

HYDRAULICS. 

CX. The science of Hydraulics teaches how 
to estimate the velocity and force of fluids in mo- 
tion. Upon the principle of this science all ma- 
chines worked by water are constructed, as en- 
gines, mills, pumps, fountains, &c. 

CXI. It is well known that water can be set 
in motion merely by its own gravity ; as when 
it is allowed to descend from a higher to a low- 
er level, either perpendicular or down an in- 
clined plane. And by an increased pressure, as of 
the air, or by removing the pressure of the at- 
mosphere, it will rise above its natural level. 

In the former case it will seek the lowest situation ; in the lat- 
ter, it may be forced to almost any height. 

Obs. 1. Water, by the process of evaporation, is made to as- 
cend into the atmosphere in the form of vapour ; afterwards it de- 
scends in the form of rain, snow, or hail. Of that which falls on 
the land, part is absorbed by the roots of vegetables, and part de- 
scending into the earth, forms subterraneous streams which break 
out in springs. 

Obs. 2. Any part of a fluid at rest presses, and is pressed, equally 
in all directions. For each particle is disposed to give way on 

251. What were the results of Mr. Robertson's experiments on the 
Specific gravity of nidi ? 252. Whnt is the specific gravity of living 
men ? 2.34. What does the science of Hydraulics teach 7 254. What 
kinds of useful machinery are constructed upon theprinciple of this sci* 
ence '.' 255. What are the different ways in which water can be put in 
motion? 256. What are the directions in which fluids press? 



HYDRAULICS. 79 

the slightest difference of pressure ; consequently, it presses 
equally in all directions. Hence the lateral pressure of a fluid, is 
equal to the perpendicular pressure. And this is one of the most 
curious properties of fluids, and can be conceived to arise only 
from the extreme facility with which the component particles 
move among themselves. 

CXII. In drawing off water, spirits of wine, 
and other fluids from vessels which cannot rea- 
dily be moved, an instrument, called a Syphon, 
is frequently used. 

Obs. 1. This instrument consists of a bended tube, the legs of 
which are of unequal lengths. The shorter leg is immersed in 
the liquor to be drawn off, and the pressure of the air being ta- 
ken from that part of the surface of the liquor within the tube, the 
licmor will rise above its natural level in the vessel, and will flow 
oft through the longer leg. 
Fig. 33. 

Obs. 2. A syphon is used b}* filling it 
with water or some other fluid, then stop- 
ping both ends with the finger, and in this 
state immersing it in the vessel. The fin- 
gers being removed, the water flows out 
of the longer leg, by its own gravity, and 
afterwards by the pressure of the atmos- 
phere on the liquor in the vessel, Fig.33. 
Obs. 3. Intermitting springs may, in 
their operations be explained, upon the principle of the syphon ; the 
water flowing through the natural pipes from reservoirs of water 
in the earth, which fill only at certain seasons. Perennial 
springs flow from unfailing reservoirs so elevated as to occasion a 
constant pressure. 




Fig. 34. 







Illus. If C be a cavern 
in a mountain, receiving 
water which escapes by 
the channel ABC, it is 
evident that it will flow 
only when it rises in D to 
the level NBD.JFV. 34. 

CXIII. Thevelo- 
W citv with which wa- 



257. In what proportion is the lateral to the perpendicu ar pressure 
of a fluid? 25?. What is one of the most curious propei-ties of fluids; 
and how can it be accounted for ? 259. What are the construction and 
use of a Syphon ? 263. Illustrate the principle of the syphon. 261. Up- 
on what principle may intermitting springs be explained? 262. What 



oU HYDRAULICS. 

ter spouts out at a hole in the side or bottom of a 
vessel, is in proportion to the square-root of the 
distance from the hole below the surface of the 
water ; and the pressure of water against the sides 
of a vessel is as the square of the depth. 

Exam. 1. If at the distance of one foot from the surface, the 
velocity is 1, an other hole four feet from the surface, would give 
the velocity of 2, and at 9 feet deep there would be a velocity of 3, 
2 and 3 being the square-roots of 4 and 9. 

2. If a vessel be three feet deep and filled with water, the pres- 
sure upon the sides of the first foot will be 1, on the first and sec- 
ond will be 4, and on the whole side it will be 9. 

Obs. 1. Let two vessels of a cylindric or prismatic form, be set 
up perpendicularly to the horizon, having their bases, heights, and 
orifices equal; if one of them be kept constantly full, by a supply 
of water, it will discharge double the quantity of water which 
would be discharged in the same time by the other while emptying 
of itself, without being supplied by water. 

2. A stream of any fluid which spouts obliquely, forms a para- 
bola. For, each drop in a stream of fluid, spouting obliquely, is 
a heavy body projected obliquely by the force or pressure which 
drives it out of the aperture. Therefore, every drop of the stream, 
that is, the whole stream, forms a parabola. 

CXIV. In consequence of the pressure of fluids, 
and the facility with which they rise to their 
first level, fluids may be conveyed over hills and 
valleys in bent pipes, to any height which is not 
greater than the level of the spring from which 
they flow. 

Obs. All water finds its own level. But from ignorance of this 
single principle, the ancients constructed vast aqueducts across 
valleys to convey water across them ; whereas, the moderns ef- 
fect the same purpose by means of wooden, leaden, iron or stone 
pipes. 

CXV. Fountains are formed upon the same 
principle ; if, near the bottom of any vessel, a 

is the proportionality existing between the velocity of liquids, issuing 
from an orifice in the side of a vessel, and the distance of the orifice 
below the surface 1 263. How may this proportionality be exemplified 1 

264. On what principle may water be conveyed over hills in bent pipes? 

265. On what principles are fountains formed ? 



HYDRAULICS. 81 

small pipe bending upwards be fastened, the 
water will spout out through the pipe, and rise 
nearly as high as the surface of the water in the 
vessel. 

CXVI. The Common Pump consists of a pipe 
open at both ends, in which is worked a movea- 
ble piston, that fits the bore exactly, and is pro- 
vided with valves. 

Fig. 35. 

lllus. Fig. 35 is the representation of a common 
pump ; a b is called the barrel which contains the 
piston; b d the pipe communicating with the wa- 
ter below. At the junction of these two parts, 
there is a fixed valve or little trap-door, d, opening 
upwards. 

The mode of operation is as follows : the nart 
d is fixed in water, and the piston c h is to be close 
down upon the valve d. In drawing up the piston 
from d to c, a vacuum of air is formed in that space, 
consequently, the air in the rest of the pipe from 
d to b will force its way through the valve d, and 
fill the part which had been exhausted ; it will, 
therefore, be rarer than before, and not being 
c^^fSk -^- equivalent to the pressure of the atmosphere upon 
the water at d, in which the pump is immersed, 
the water will be forced or pressed up into the pipe 
as far as or, until the upper air within becomes as dense as before. 

Upon depressing the piston a second time, the same effect is 
produced, till at length the water is forced, by the pressure of the 
air o, up into the pipe above d. When the piston now descends, 
it is forced into the water, which, as it can not repass through the 
valre d, must, therefore, rise above the piston by passing through 
its valve c ; and when the piston is next raised, ail the wafer 
above it will be lifted up, and will run off by the pipe. 

CXVII. The Forcing Pump consists of a bar- 
rel, a plunger, and two fixed valves, that should 
be air-tight, and so disposed as to let the water 
freely rise, but prevent its return. 

266. What are the construction, mode of operation, principle, and use 
of the Common Pump ? 267. What are the different parts of a For- 
ting Pump, and the principles upon which it acts ? 





HYDRAULICS 

Ulus. In Rg. 30, a b is thebarrel ; c a solid pis- 

| ton or plunger ; at (/, is one valve opening upwards, 

f the other is in the branching pipe 8. When the 

piston is first moved upwards in the barrel, the air 

below will be rarefied and the water rise up in b; 

lift and by re eated strokes of the piston, the water 

1 will he brought up between the fixed valve d and s. 

It can not, therefore, descend by d, but must 

make its way through the upper valve at s, which 

shuts immediately after the water has passed, by 

its own weight. 

v is a strong air-vessel closed at the top by a small 
pipe, that reaches nearly to the bottom. The wa- 
ter is forced along the rising pipe s, gets into the 
vessel, and rises above the lower partof the pipe. 
The air, which is above the water in the vessel, be- 
ing confined, and condensed into a smaller bulk 
than its natural space, presses by its elasticity upon the surface 
of the water, and forces it violently up the pipe in a continual 
stream. This is the principle of the Engine for extinguishing 
fires. 

CXVIII. The water is raised by pumps in 
consequence of the elasticity or pressure of the 
atmosphere ; it can be raised only about 33 feet, 
because the force of the atmosphere is equal on- 
ly to a column of water 33 feet high. 

Obs. 1. The forcing pump is unlimited in regnrd to the height 
to which it can throw water. The air-vessel added to the forcing 
pump, gives the water a more equable stream. 

2. A constant stream may be produced by two barrels with pis- 
tons moving up and down alternately, as is the case in many 
pumps. 

CXIX. The Steam Engine consists of a 
large cylinder or barrel, in which is fitted a solid 
piston like that of a forcing pump. Steam is 
thus supplied from a large boiler, which, in forc- 
ing up the piston, instantly opens a valve, through 
which cold water rushes, on the principle of the 
common pump. 

26S. What is the principle of the Fire Engine 7 269. How high can 
Water be raised by means of a common ump, and on what principle t 



HYDRAULICS. 



83 



Other steam is then introduced above the pis- 
ton, which forces it down again, and drives the 
water out of the pipe with immense force. 

Steam then raises the piston again, and again 
makes it fall ; and by this alternate motion, the 
grandest operations are performed. The action 
of the piston moves up and down a large beam, 
and this be;mi communicates to other machine- 
ry, the power of 100 or 200 horses ! 

Obs. l. The power of some of the steam engines, constructed 
by Messrs. Boulton and Watt, is thus described, as taken by actual 
^jjeriment. An engine having a cylinder of 31 inches in di- 
ameter, and making 1? double strokes per minute, performs the 
work of 40 horses, working night and day, for which 3 relays, or 
120 horses must be kept, and burns 11,000* pounds of Staffordshire 
coal per day. A cylinder of 19 inches, making 25 strokes. 4 feet 
each, per minute, performs the work of 12 horses working con- 
stantly, and burns 3,700 pounds of coal per day. These engines 
will raise more than 20,000 cubic feet of water, 24 feet high, for 
every 100 weight of good pit-coal consumed by them. 



Fig: 37. 




2. Fig. 37 represents the 
common steam engine. A is 
the iron cylinder, containing 
the piston which works up and 
down, and moves the beam and 
the resistance B. The steam 
passes from the boiler, at the 
pipe D, into the cylinder under 
the piston, raises it, and the 
jet at C throws cold water, 
condenses the steam, and al- 
lows the piston to fail again. 



270. What is the construction of a Steam Engine; and on what prin- 
ciple does it act 1 What example is given, as illustrative of the powers 
ofa steam engine 1 



84 



HYDRAULICS. 



Fig. 38. 




3. Fig. 38 represents Watt's improved engine. The principle is 
the same as the preceding-, hut the economy is greater. The 
steam which is below the piston escapes into the condenser A, by 
the cock B, which is opened by the rod C, and at the same time 
the steam is admitted by the cock D into the upper part of the 
cylinder; when the piston has descended, the cocks E and Fact 
in a similar manner in letting out the steam from above, and ad- 
mitting it below the piston. The jet is supplied by the water of 
the cistern G, which is pumped up at H, from a reservoir ; it is 
drawn out, together with the air, that is extricated from it, by the 
air-pump I, which throws it into the cistern K, whence the pump 
L raises it to the cistern M ; and it enters the boilers through a 
valve which opens whenever the float W descends below its pro- 
per place. The pipes O and P serve also to ascertain the quan- 
tity of water in the boiler. The piston rod is confined to a motion 
nearly rectilinear by the frame Q,. The fly wheel R is turned by 
the Sun and planet wheel S T, and the strap U turns the centrifu- 
gal regulator W, which governs the supply of steam by the valve 
or stop cock X. 
■ \ • 

272. Give a description of Watt's improved engine. 



rNEUMATICS. 85 

4. Steam engines have been advantageously applied lately to 
impel vessels in smooth waters, as rivers, canals, &e. 

CXX. Water has long been employed as the 
moving power of machinery, particularly of mills. 
When so employed, it has usually been applied 
to the circumference of wheels, from the axis or 
axletree of which the power is conveyed to the 
other parts of the machine. 

Obs. 1 . When the water is conducted over the summit of a 
wheel, and introduced into buckets ranged round its circumfer- 
ence, so as to put the wheel in motion principally by its weight 
in the buckets, the wheel is denominated an Overshot Wheel. 

2. When the water, after having acquired a considerable velo- 
city by its descent along an inclined plane, is brought under the 
wheel in such a manner as to strike plane surfaces or float boards, 
arranged round its circumference, so as to put the wheel in motion 
by its impulsive force, it is called an Undershot Wheel. 

Besides those above-mentioned, there are several other kinds of 
water wheels, such as the Breast Wlieel, Tub Wheel t Spiral or 
Screw Wheel, &c. It appears, however, from experiments, that 
an overshot wheel is the most powerful ; its effect being nearly 
double the effect of an undershot wheel. But for a minute account 
of these and various other hydraulic machines, the inquisitive 
reader is referred to such scientific publications as treat exten- 
sively of those branches of Natural Philosophy. 

PNEUMATICS. 

CXXI. The science of Pneumatics treats of 
the mechanical properties of the atmosphere, and 
of elastic or aeriform fluids in general, such as 
their weight, pressure, density, elasticity, and 
compressibility. 

CXXIL The whole globe of the Earth is 
every where surrounded by a vast mass of fluid, 
called air, which, though perfectly invisible to us, 

273- To what useful purposes have steam engines been applied 1 274. 
In what, manner has water been long employed as the moving power of 
machinery'? 275. What is an Overshot Wheel? 276. What is an Under- 
shot Wlieel? 277. Which is the more powerful of the two ? 278. What is 
the object of the science of Pneumatics i 279. What is the atmosphere i 

S 



86 PNEUMATICS. 

is however manifested in various ways. This fluid 
in which the Earth is involved, and in which 
we live, tills up the space from the surface of the 
Earth to the height of several miles above it; 
and the whole mass of it together with the clouds 
and vapours and some mixture of other sub- 
stances that float therein, is called the Atmo- 
sphere. 

Obs. 1. The air or atmosphere is a light, attenuated, elastic 
body, and is absolutely necessary not only to animal life, but also 
to combustion, vegetation, and to other natural processes. It is a 
very heterogeneous mixture, being composed of vapours of va- 
rious kinds ; it consists, however, of two great principles, called 
Oxygen and Hydrogen , in the proportions of 80 parts of oxygen, 
and 20 parts of hydrogen, in 100. 

2. The height to which the atmosphere extends has never been 
correctly ascertained. From calculations founded on the pheno- 
mena of refraction and reflection, its height has been supposed to 
be about 45 miles ; for at a greater height than 45 miles, it ceases 
to reflect the rays of light from the Sun. 

' CXXIIT. The atmosphere, when clear of va- 
pours and exhalations, is invisible, because it is 
perfectly transparent. Of its existence, however, 
as a fluid substance, we have sufficient evidence 
from its resistance to bodies moving through it, 
and from the force with which it acts when it is 
in motion and produces what we call Wind. 

Obs. The invisibility of air is what suggests the vulgar idea of 
its being nothing. But it should be recollected that air is perfectly 
transparent, and that transparent bodies, or such as admit the 
rays of light to pass freely through them, can not be seen. 

'Exj). 1. The existence of the air may be ascertained by swing- 
ing the hand edgewise swiftlv up and down; also by moving a 
shingle or piece of board swiftly through the air — first, side fore- 
most and then edgewise; and likewise by letting fall a piece of 
paper when it is expanded, and again after it is rolled to a close 
ball, the resistance of the air will be readily perceived ; as such 

2^0. What ere the uses of the atmosphere, and of what, is it composed? 
2 1. To what height abovethe Earth does the atmosphere extend? 2-2. 
in what manner may the existence of air be ascertained? 



PNEUMATICS. 87 

motion gives the idea of separating the parts of some resisting 
medium. 

2. Any swift motion, as of a stick, or whip, or fan, proves the 
existence of air as a resisting medium. 

CXXIV. Air is about 830 times lighter than 
water ; but the whole atmosphere presses on all 
sides like other fluids, upon whatever is immersed 
in it, and in proportion to the depths. 

Obs. 'As a fluid, the air resembles water in some of its proper- 
ties, while it differs essentially from it in others. Like water, it 
possesses gravity or weight, and pressure, though in a different 
degree ; for it is preserved. at the surface of the Earth by the force 
of gravity, and revolves with it round the Sun. But the air differs 
from other fluids or water, chiefly in being extremely compressi- 
ble and wonderfully elastic. 

Exam. 1. The pressure of the air upon any body is proportion- 
ed to the distance of that body from the upper surface of the atmo- 
sphere. It is known, by experiments made with barometers, that 
the pressure of the atmosphere is greater in valleys than on high 
mountains. 

When the difference of elevation is small, and the ascent gradu- 
al, the change of weight is scarcely perceptible, except by means 
of a barometer ; but when great heights are attained suddenly, the 
transition causes unpleasant sensations. Respiration becomes 
frequent, difficult, and paii.ful ; and at the height of five miles 5 
respiration is supposed to be impossible. 

2. The pressure of the air may be thus shown ; — cover a wine 
glass completely filled with water, or wine, with a piece of writing 
paper ; then place the palm of the hand over the paper, so as to 
bold it tight and accurately even. The glass may then be turned 
upside down, and the hand removed without the water running 
out. The pressure of the air upon the paper sustains the weight 
of the water. 

3. It is the pressure of the atmosphere which sustains the mer- 
cury in the barometer tube. On ascending a mountain the mer- 
cury sinks. This shows that a part of the pressure is taken off. 

On the surface of the Earth, water boils at 212 degrees; on 
Mount Blanc, it boils at 187 degrees. These and many other 
experiments show, the higher we ascend from the surface of the 
Earth, the less is the atmospheric pressure. 

283. How much lighter is air than water? 2S4. What points of analo- 
gy and difference are observable between air and water?. 2S5. How is 
the pressure of the air shown % 2S6. What is said of respiration in ele- 
vated situations'? 2>7. How is it known that the pressure of the atrno. 
ephere decreases upwards 1 



88 PNEUMATICS. 

4. The weight of the air varies with its temperature and den- 
sity. It is heavier in clear, cold weather, than when the weather 
is warm and cloudy. Its usual weight beinsr about 830 times less 
than that of water, a cubic foot of air will consequently weigh 
something more than one ounce ; a cubic foot of water weighing 
about 1000 ounces. 

The pressure of the atmosphere on every square inch of the 
Earth's surface, is equal to that of a column of mercury one inch 
square and about 30 inches in height; which is equal to a similar 
column of water 33 feet in height.* 

Hence the pressure of the atmosphere on every square inch of 
the Earth's surface, is computed to be about 15 pounds avoirdu- 
pois ; about 2160 pounds, or little more than one ton on every 
square foot. The pressure on the whole surface of the Earth, 
therefore, is more than 11865803548262400000 pounds— equiva- 
lent to that of a globe of lead 60 miles in diameter. 

Supposing the whole surface of a man's body to contain 15 
square feet, which is pretty near the truth, a man of ordinary size 
must sustain a weight of 15 times 2160, or 32,400 pounds, that is, 
more than 1 6 tons weight for his ordinary load, when the atmo- 
sphere is at a medium weight. By this enormous pressure, we 
should undoubtedly be crushed in a moment, if all parts of our 
bodies were not either filled with air or some other elastic fluid, 
the elasticity of which is just sufficient to counterbalance the 
weight of the atmosphere. The atmospheric pressure is some- 
times increased, at other times it is diminished, differing about a 
ton and a half on the human body, according to the fluctuations 
and changes in that great elastic fluid which we breathe. 

CXXV. The air can be compressed into a less 
space than it naturally occupies. 

Exp. 1. Take a glass tube open only at one end, and it is of 
course full of air. Plunge the open end into a bowl of water, and 
we see the water rises an inch or so in the tube ; the air, therefore, 
which before filled the whole length of the tube, is compressed 
by the water into a smaller space. 

2. Take a cork swimming on a basin of water, cover it with an 
empty glass tumbler, which force down through the water. The 
cork evidently shows, that the surface of the water within the 

238. What is the pressure of the atmosphere on every square inch of 
the Earth's surface 1 — On every square foot? — And on the entire surface 1 

289, What weight of air is a common sized man supposed to sustain 1 

290. Why does not so enormous a weight crush us to atoms 1 

* It is in consequence of the pressure of the atmosphere, that water is 
forced up into a common pump through its valves. Therefore a pump 
whose stock is more than 33 feet in length, between the valves, can not 
have the water conveyed to the spout by the force of the air ; but all 
above that height must be raised by means of other mechanical powers. 



PNEUMATICS. 89 

tumbler is not on a level with the surface without. This experi- 
ment proves that air is a body which prevents water from occupy- 
ing the same space with itself; it proves also that the air is 
compressible, because the water does not ascend 'in the glass. 

CXXVI. The air is of an elastic or expanding 
nature, and the force of the spring is equal to 
what is commonly called its weight. The spring, 
however, operates in all directions, and is as pow- 
erful in small as large bulks. 

Exp. 1. Fill a bladder with air by blowing into it, and in this 
state the bladder is highly elastic ; it proves also that air is as 
much a substance as wood or metal, for no force can, without 
breaking the bladder, bring the sides together, though the parts 
of an empty bladder may be squeezed into any shape. 

2. Opena pair of common bellows in the usual manner, and 
then stop the nozzle securely, and no force can bring the parts 
together, without first unstopping the nozzle, or bursting the lea- 
ther, an other proof that air is a solid substance. 

CXXVIL "When air is in motion it constitutes 
what we call Wind ; which is nothing more 
than a current of air, varying its force, according 
to the velocity with which it flows. 

Obs. As fishes are surrounded by water, and live and move in 
that fluid, so are we, human beings, and all other land animals, 
surrounded by air, and live and move in air. A fish which is taken 
out of the water, will die in a short time ; and a human being, 
or any animal taken out of the aerial fluid, will in general die 
much sooner. 

As the progressive motion of water from one place to an other, 
is called a current of water ; so the progressive motion of the at- 
mospherical air is called, in general, windy which, according to 
the different velocities of that fluid, is more particularly specified 
by the appellations, breeze, gentle wind, gale, hurricane, &c. 

CXXVIII. Air-pumps are machines con- 
structed for the purpose of exhausting the air 

291. What is said concerning the compressibility of air 1 292. What 
is to be understood by the elasticity, or expanding nature of the atmos- 
phere 1 293. How is the elasticity of the atmosphere illustrated 1 294. 
What is FFmd ? 235. What is an Air-pump 7 

8* 



90 PNEUMATICS 

from certain glass vessels adapted to the purpose 
of experiments on air.* 

lllus. Fig. 39, represents one of the most convenient air- 
pumps. 

Fig. 39. 




In this figure, a a represents two brass barrels, each containing 
a piston, with a valve opening upwards. The pistons are worked 
by means of the winch 6, which moves them up and down alter- 
nately. On the wooden frame d e, there is a brass plate g, ground 



*The air-pump was first invented by Otto Guericke, a burgomaster of 
Magdeburgh in Germany, about the year 1654. This pneumatic engine 
is one of the principal instruments which have, since the middle of the 
17th century, contributed to the rapid advancement of Natural Philoso- 
phy, by affording the means of verifying what had been advanced and 
conjectured by several learned and ingenious persons concerning the 
atmosphere, of producing a vacuity or vacuum in which a great number 
of curious experiments may be performed, and of ascertaining a vast 
number of new and interesting facts. Soon after the invention of the 
air-pump, Guericke's contrivance was imitated and greatly improved 
in England, by the celebrated Mr. Boyle und Dr. Hook, and subsequently 
by various ingenious persons, among whom we may mention Mr.Hawkes- 
hro, Gravesande, Mr. John Smeaton, the celebrated engineer, Mr. Haas, 
Cuthbertaon, Lavoisier, and lastly, Mr. Prince of Boston Mass. 






PNEUMATICS. 91 

perfectly flat and even ; and also a brass tube communicating 
with the two cylinders beneath, and the cock i, and opening into 
the centre of the brass plate at./*. 

k is the glass receiver which is to be exhausted of air, and is 
made to fit very accurately on the brass plate, particularly when 
a wet piece of leather is laid between them. 

Having shut the cock i } the pistons are worked up and down, 
and the air is drawn from the glass receiver through the pipe, 
and is suffered to escape ; when the piston is forced down 
the air rises through it, because the valve opens upwards, but it 
js prevented from returning into the vessel for the same reason. 
The air being gradually exhausted from the receiver, it becomes 
immoveably fixed by the pressure of the surrounding atmosphere. 

Upon opening the cock i, the air rushes again violently, and 
•with a noise into the receiver. 

CXXIX. The Air-pump is the grand ma- 
chine by which experiments on air are made. 
JBy its means the following important properties 
<of air are demonstrated. 

i. The air has weight. 

Exp. 1. The air being exhausted by an air-pump, from a glass 
jeceiver, the receiver will be held fast by the pressure of the ex- 
ternal air. 

2. If a small receiver be placed under a larger, and both be ex- 
hausted, the larger w 7 ill be held fast, w T hile the smaller will be ea- 
sily moved. 

3. If the hand be placed upon a small open vessel, in such a man- 
ner as to close its upper orifice, it will be held down with great 
force. 

4. The upper orifice of an open receiver being closely covered 
with a piece of bladder, upon exhausting the receiver, the bladder 
will be pressed till it burst, and in the same situation, a thin plate 
of glass will be broken. 

5. Pour mercury into a wooden cup, closely placed upon the up- 
per orifice of an open receiver ; when the air beneath is exhaust- 
ed, the pressure of the external air w T ill force the mercury through 
the w r ood, audit will descend in a shower. 

6. On a transferrer. let the air be exhausted from a long receiver, 
then let water be admitted through a pipe, by means of a cock ; the 
water will rise in a jetd/eau. 

7. Let the air be exhausted from a glass vessel, and by means 
of a cock, let the vessel be kept exhausted ; w r eigh the vessel 
whilst it is exhausted, and when the air is to be re-admitted, the 

296. What are the construction and principle of an air-pump 1 297. 
JVhat is said of the invention and improvements of air-pumps ? 




92 PNEUMATICS. 

difference is the weight of so much air as the vessel contains; 
w bicfa difference will be about 305 grains for a thousand cubic 

inches. 
Fig. 40. 

8. Into the receiver a, Pig: 40, put a small vessel of 
quicksilver, and through the collar of leather as at 
o, suspend a glass tube, closed at the upper end, 
over the quicksilver. The apparatus thus situated 
is to be placed on the brass plate of the air-pump, 
and the air completely exhausted from the receiver ; 
the tube is then to be let down into the quicksilver, 
which will not rise in it as long as the receiver 
continues empty; but as soon as the air is re-ad- 
mitted, all the surface of the quicksilver is pressed 
upon bv the air, except that portion which lies 
above trie orifice of the tube ; it will therefore rise 
in the tube, until the weight of the elevated quick- 
silver presses as forcibly on that part of it which 
lies beneath the tube, as the weight of the air does 
on every other equal portion without the tube. 

9. A common experiment among bjys is on the same principle, 
^ake a piece of thick spongy sole leather, cut it into a circular 
form, and through the centre pass a string ; wet it thoroughly, 
and place it flat on a smooth surface ; then try to pull it up in a 
perpendicular line. A vacuum is formed in the centre, while the 
edges are pressed down by the weight of the atmosphere. In 
this way, a smooth stone of many pounds weight may be lifted. 

Obs. 1. Hence the pressure of the atmosphere on or near the 
surface of the Earth, is known ; the weight of any column of air 
being equal to the weight of the column of mercury, of the same 
diameter, supported in the barometer. And, since the height of 
this column varies with the weight of the atmosphere, between 
23 and 31 inches, equal to 33 or 34 feet of water, the varieties in the 
weight of the atmosphere are known by the barometer. The 
most usual altitude of the barometer, in London, is between 28 
and 31 inches, but it is seldom seen below 23£ or above 30£ 
inches. y 

2. In calm weather, when it is inclined to rain, the mercury is 
commonly low. In serene settled weather, the mercury is gene- 
rally high. During very great winds, though unaccompanied by 
rain, the mercury sinks lowest of all. 

ii. The air presses equally in all directions. 

Exp. 1. If a glass vessel be filled with water, and covered with 
a loose piece of paper, on inverting the glass, the water will be 
kept from falling Dy the upward pressure of the air. 

298. What experiments prove that air has weight ? 20*.). What is said 
of the variation of the mercury in the barometrical tube? 



PNEUMATICS. 93 

2. If a vessel be perforated in small holes at the bottom, but 
closed at the top, the upward pressure of the air will keep the 
water within the vessel ; as will appear by successively stopping 
and unstopping a small hole inlhe top of the vessel, or in drawing 
beer from an air-tight cask. 

3. Two brass hemispherical cups put close together, will, when 
the air between them is exhausted, be pressed together with con- 
siderable force. 

This corresponds with the apparatus constructed by the celebra- 
ted Otto Guericke, by means of which that distinguished philoso- 
Sher was enabled to resist the action of twelve coach horses 
rawing in opposite directions. 

4. A syringe being fastened to a plate of lead, and the piston of 
the syringe being drawn upwards with one hand, whilst the lead 
is held in the other, the air, by its upward pressure, will drive 
back the syringe upon the piston ; whereas, il the loaded syringe 
be hung in a receiver, and the air being exhausted, the syringe and 
lead will descend ; but upon re-admitting the air, they will again 
be driven upwards. 

5. If a thin glass vessel, whose aperture is closed, be placed 
under the receiver of an air-pump, and the air exhausted from the 
receiver, the vessel will be broken by the pressure of air within. 

in. The air is an elastic fluid, or capable of 
compression or expansion. 

Exp. 1. A blown bladder, pressed with the hand, will return 
into the form which it had before the pressure. 

2. An empty bladder put under a receiver, when the external 
air is exhausted, becomes extended by the elasticity of the inter- 
nal air. 

3. A bladder suspended within the receiver, with a small weight 
hanging from it, which touches the bottom, when the external air 
is exhausted, by the expansion of the internal air, will raise the 
weight. 

4. The bladder berns* put into a box, and a weight laid upon the 
lid, the lid, on exhausting the air, will be raised. 

5. Put a shrivelled apple into the exhausted receiver, and it will 
become distended or swelled to its original compass, by the ex- 
pansion of its internal air. 

6. Put an egg into the exhausted receiver with the small end 
broken ; the expansion of the air contained in the opposite end 

j will expel the contents from the shell. 

7. On green vegetables and various other substances, placed in 
a vessel of water under a receiver, whilst the air is being exhausted, 
bubbles will be raised by the expansion of the internal air. 

300. How is it demonstrated that air presses equally in all directions ? 
' SOI. How is the elasticity of the air proved ? 




94 PNEUMATICS. 

8. Set a cup of water in the exhausted receiver of an air-p 
and when the air is nearly exhausted from the receiver, the wa- 
ter will have all the appearance of boiling, in consequence of the 
expansion of the air and its arising out of the water. 

9. Beer, a little warmed, will, from the same cause, whilst the 
internal air is being exhausted, have the appearance of boiling. — 
Thus it may be shown that air is contained in water, animals and 
vegetables. 

10. To a cylindrical piece of wood, fasten a small piece of lead, 
so as to make it just specifically heavier than water, and place 
it in a vessel of water under a receiver, upon exhausting the air 
the wood will swim ; some particles of air escaping from the 
wood and thereby diminishing its specific gravity. 

11. If a tube, closed atone end, be inserted at its open end, 
in a vessel of water, the fluid in the tube will not rise to the level 
of the water in the vessel, being resisted by the elastic force of 
the air within the tube. On this principle the diving bell is 
formed. 

Fig. 41. 

afl 12. This bent tube abed, Fig. 41, is open at 

both ends. I have poured mercury into the tube so 
as to rise in both sides of the tube c and b; the part 
from c d, is full of air at the common density ; I stop 
j up d, so as to make it air-tight, and pour mercury 
* into a, so that the column of mercury a 6, shall be 
e equal in length to the height at which it stands in 
C the barometer at the time. The air in the shorter 
leg will now be compressed by the weight of the at- 
mosphere, and also with an additional equal weight 
of a column of mercury ; and the mercury in the shorter leg will 
be risen to e, and a e, is the only half c f d c; that is, the pressure 
of a double atmosphere compresses the air to half the space which 
it naturally occupies. If an other equal column of mercury, were 
added to tie length a b, the air in d c, would be reduced into one- 
fourth the spacelhat it formerly occupied. 

302. What experiments serve to demonstrate this elasticity ? 



PNEUMATICS. 



95 



Fig- 





13. In Fig. 42, a is a strong copper vessel, having a 
tube that screws into the neck ol it, so as to be air- 
tight, and long enough as nearly to reach the bottom ; 
x is the handle of a stop-cock. Having poured some 
water into the vessel, and screwed in the tube, the con- 
densing syringe is to be adapted, and the air con- 
densed. The stop-cockis to be shut, while the syringe 
is unscrewed, then, on opening the cock, the air, by 
its great density acting upon the water in the vessel, 
will force it out in a jet to a considerable height. This 
is called the Artificial Fountain. 

iv. The elastic spring of the air is 
equivalent to the force which compresses 
it. 

Exp. Let the air be exhausted from an open tube, 
whose lower part is inserted in a vessel containing a 
small quantity of mercury , and let the air withiu the 
vessel be prevented from escaping; this air by its elasticity, will 
force the mercury up the tube nearly to the height to which it 
would be raised by the pressure of the atmosphere. 

Obs. If the spring with which the air endeavours to expand it- 
self when it is compressed, were less than the compressing force, 
it would yield still farther to that force ; if it were greater, it would 
not have "yielded so far. Therefore, when any force has compress- 
ed the air so that it remains at rest, the spring of the air arising 
from its elasticity must be equal to the pressure. 

v. The elasticity of the air is increased by 
heat. 

Exp. 1. To the bottom of a hollow glass ball, let an open bended 
tube be affixed. Let the lower part of the bended tube and a part 
of the ball be filled with mercury ; the external surface will be 
pressed by the weight of the atmosphere ; and the internal surface 
will be equally pressed by the spring of the air enclosed within the 
vessel. If the ball be immersed in boiling water, the increased 
elasticity of the included air will raise the mercury in the small 
tube. The same may be shown by immersing in boiling water, a 
tube, closed at one end, into which a small quantity of mercury 
has been admitted, inclosing a portion of air within the tube. 

Obs. Wind is no other than the motion of the air upon the sur- 
face of our globe, being a stream or current of that fluid, as a river 
is of water ; and is occasioned by heat, eruptions of vapours, con- 

333. What are the construction, principle, and mode of operation, of 
zx\^A.ir -fountain 7 304. What is said of the elastic spring of the air ]— 
305. What effect does heat have on the elasticity of air 1 



96 PNEUMATICS. 

densations, rarefactions, the pressure of clouds, the fall of raing, 
or some other phenomenon that disturbs the equilibrium of the 
air ; for; as the air is a fluid its natural state is that of rest, "Which 
it endeavours always to keep or retrieve by a universal equilibri- 
um of all its parts. When, therefore, this natural equilibrium of 
the atmosphere happens by any means to be destroyed in any part, 
there necessarily follows a motion of all the circumjacent air to- 
wards that part to restore it; and this motion of the air is what 
we call wind. 

Hence, with respect to the place in which the equilibrium of the 
air is disturbed, we see that the wind may blow from every point 
of the compass at the same time ; and those Who live northward 
of that point have a north wind ; those who live southward, a 
south wind ; and so of the rest. But those who live on the spot 
where all these winds meet and interfere, are oppressed with tur- 
bulent and boisterous weather, whirlwinds and hurricanes, with; 
rain, tempests, lightning 1 , thunder, &c. 

Although many are the particular causes which produce wind? 
by interrupting the equipoise of the atmosphere ; yet the princi- 
pal and most general cause of wind is the heat communicated by 
the Sun, in consequence of which the atmosphere is heated over 
one part of the Earth more than over an other. For, in this case^. 
the warmer air being rarefied, becomes specifically lighter tharc 
the rest ; it is therefore overpoised by it and caused to ascend, as 
lighter fluids always do, till it reaches some elevated part of the 
atmosphere, of the same density, where it can remain at rest, — 
the upper parts of it diffusing themselves every way over the top' 
of the atmosphere ; while the neighbouring inferior air rushes in 
from all parts at the bottom ; which it continues to do till the 
equilibrium is restored. Upon this principle, it is, that most 
winds may be accounted for. 

Hence it is that in all parts over the torrid zone, the air being 
more rarefied by a greater quantity of the solar rays, is much light- 
er than in the other parts of the atmosphere, and most of all, over 
the equatorial parts of the Earth. And since the parts at the equa- 
tor are most rarefied which are nearer to the Sun ; and those parts 
are, by the diurnal rotation of the Earth eastward, continually mo- 
ving to the west ; it follows, that the parts of the air which are 
on the west side of the point of greatest rarefaction, and by 
flowing towards it, meet it, have less motion than those parts on 
the east of that point, which follow it; and therefore the motion of 

306. What experiments demonstrate that the elasticity of air is in- 
creased by heat? 307. What explanation may be given of the pheno- 
mena of wind ? 303 What circumstances are remarked with respect to 
the motion of air in producing wind ? 309. What is the consequence 
when winds from different quarters meet and interfere ? 310. How is 
the air put in motion so as to produce wind ? 311. What are the princi- 
pal causes of wind ; and in what manner do they operate? 312. What 
regular winds prevail about the equatorial regions of the Earth 1 



PNEUMATICS. 97 

the eastern air would prevail against that of the western air, and 
so generate a continual east wind, if this were all the effect of 
that rarefaction. But we are to consider, that, as all the parts of 
the atmosphere are so greatly rarefied over the equator, and all 
about the poles greatly condensed by extreme cold, this heavier 
air from each of the poles is constantly flowing towards the equa- 
tor, to restore the equilibrium destroyed by the rarefaction and 
levity of the air over those regions ; hence, in this respect alone 3 
a constant north and south wind would be generated. 

Now, it is easy to understand, that by a composition of these 
two directions of the air from the east and north, a constant north- 
east wind will be generated in the northern hemisphere, and a 
constant southeast wind in the southern hemisphere, to a certain 
distance on each side of the equator all round the Earth. And this 
case we find to be verified in the general trade winds, which con- 
stantly blow from the northeast and southeast within about thirty 
degrees on each side of the equator, "where those parts are over the 
jpen ocean and not affected by the reflection of the sunbeams from 
the heated surface of the land ; for in this case the wind will always 
set in upon the land, as on the coast of Guinea and other parts of 
the torrid zone, we know it does. It must however be recollected 
that though the concurrence of the east wind with those from the 
north and south, tends to produce a general northeast and south- 
east wind, yet at places near the equator, the wind blows con- 
stantly from the east, or very nearly so ; and near the thirtieth 
degree of latitude, the winds are generally more to the north and 
south. 

Winds are usually distinguished into three kinds, independent- 
ly of the names which they take, according to the point of the com- 
pass from which they blow ; — these are the Constant or Perma- 
nent winds, the Periodical, and the Variable winds. These last 
appear to be subject to no general rule. 

The Constant winds, as above described, are those which are 
found in the torrid zone, or between 30 degrees north and south of 
the equator. These are sometimes called Permanent Trade 
Winds. 

In some places within the Tropics a species of wind is observa- 
ble, which the navigators call Monsoons, or Periodical Trade 
Winds. These Monsoons, or shifting trade winds, blow regularly 
during a certain period, as six months, in one direction ; and dur- 
ing the remaiuing part of the year, they blow in the contrary direc- 
tion, or from the opposite point. These trade winds are found to 

313. Why is it, that a regular east wind prevails at and near the equa- 
tor? 314. How are the General Trade Winds occasioned? 315. To whac 
distance on each side of the equator do these winds extend? 316. What 
is said of the influence of the sunbeams reflected from the heated sur- 
face of the land 1 317. Into how many kinds are winds usually distin- 
guished ; and what are they denominated 1 318. What winds are called 
Monsoons ] 

9 



98 PNEUMATICS. 

prevail chiefly in the oceans to the south of Asia; and may be ac- 
counted for in the following manner ; — The Indian oceanis bound- 
ed on the north by Arabia, Persia, India, &c. Part of these coun- 
tries is within the torrid zone; and, therefore, while the Sun is in 
the northern hemisphere, and particularly when near the tropic of 
Cancer, these countries become very hot, and consequently the in- 
cumbent air is rarefied and ascends, the cool air then flows in from 
the sea to restore the equilibrium, and will continue to do so while 
the land is hotter than the ocean ; and hence a southwest wind is 
generated. When the Sun returns to the south side of the equinoc- 
tial, approaching the tropic of Capricorn, the northeast trade wind 
commences and continues during the remaining part of the year. 
From the position of coasts, gulfs, &c. these winds take different 
directions. 

In islands and places near the sea, and on the sea-shore of al- 
most all countries in warm climates, there is usually a gentle wind 
from the land at night, and from the sea in the day.* The land 
and sea-breezes may be explained in the same manner as the pe- 
riodical trade winds. For, during the day, the sea is not so much 
heated by the presence of the Sun as the land ; nor is it so much 
cooled during the night. Hence, in the day-time, the cooler air 
from off the sea will rush towards the land to supply the deficiency 
occasioned by the greater rarefaction of the air; thus producing 
the sea-breeze. In like manner, during the night, the air at land 
being more cooled than that on the sea, will, therefore, flow from 
the land towards the sea ; and, hence, occasion a land-breeze. 

From what has been observed, we may readily perceive, why 
the northern and southern parts of the world beyond the limits of 
the trade winds, are subject to such variety of winds. For, the air, 
on account of the smaller influence of the Sun in those parts, be- 
ing undetermined to move toward any fixed point, is continually 
shifting from place to place, in order to restore the equilibrium, 
whenever it is destroyed, by the heat of the Sun, the rising of va- 
pours or exhalations, the melting of snow upon the mountains, or 
other circumstances. So large a portion of tne atmosphere as that 

319. How is the variation of the monsoons produced ? 

320. In what manner are the land and sea breezes produced 1 321. 
What is said of them as occurring in the Mediterranean, and in the 
West Indies, SfC. 7 322. What is it that occasions the great variety of 
winds which occur in the northern and southern parts of the world, or 
in the temperate zones 1 323. What are the principal causes of the va- 
riable winds? 

* In the Mediterranean sea, the West Indies, &c. the land-breeze usu- 
ally begins at six or seven o'clock in the evening, and blows until eight 
in the morning, after which the Seabreeze begins, increasing till noon, 
and gradually dying away in the afternoon. Between the changes, there 
leriod of stillness, as between the ebbing and flowing of the tides. 
Without the Seabreeze, the heat of many places in those climates would 
be insupportable. Even in our own climate, we frequently experience, 
in calm weather, gentle breezes blowing from the sea to the land. 



PNEUMATICS. 99 

within the torrid zone, being in continual agitation, these agitations 
in an elastic fluid which yields to the slightest impression, must 
extend every way to a great distance. The air, therefore, in all 
climates, will suffer more or less perturbation, according to the sit- 
uation of the country, the position of its mountains and valleys, 
and from a variety of other causes. Hence it is easy to conceive, 
that almost every climate must be liable to variable winds. 

In some places, there are large mountains perpetually covered 
with snow ; in others, warm lakes of water, which have different 
effects upon the atmosphere ;-^such circumstances may cause fre- 
quent changes in the wind, and occasion many of its irregularities. 
—Some land is more quickly operated upon by the rays of the Sun 
than other soil near it which may contain some internal fire, or 
some substances that seem to heat the Earth in particular places ; 
so that the changes in the atmosphere do not depend wholly upon 
the heat of the Sun. 

But it is extremely difficult if not impossible to give any ade- 
quate account of irregular winds, especially of such sudden and 
violent gusts as come on at very irregular periods, and generally 
continue for a short time. They sometimes overspread an exten- 
sive tract of country ; and at other times, are confined within a 
remarkably narrow space. Their causes are by no means rightly 
understood, though they have been vaguely attributed to peculiar 
rarefactions, to the combined attractions of the Sun and Moon, 
to earthquakes, electricity, &c. They are in general called Hurri- 
canes, or they are the principal phenomena of a hurricane, that 
is, of a violent storm. 

Almost every one of those violent winds is attended with par- 
ticular phenomena, such as droughts or heavy rains, or hail or 
snow, or thunder and lightning, or several of those phenomena at 
once. They frequently shift suddenly from on quarter of the ho- 
rizon to an other, and then come again to the former point. In 
this case they are called Tornadoes. 

When the gusts of wind come from different quarters at the 
same time, and meet while moving swiftly, in a certain place, 
there the air acquires a circular, or rotatory, or screw-like motion, 
either ascending or descending, as it were, round an axis, and this 
axis sometimes is stationary, and at other times moves on in a 
particular direction. This phenomenon, which is called a Whirl- 
wind, gives a whirling motion to dust, sand, water, part of a cloud, 
and sometimes even to bodies of great weight and bulk, earrving 
them either upwards or downwards ; and lastly scatters them 
about in different directions. 

The phenomenon called a Water-spout has been attributed prin- 
cipally, if not entirely, to the meeting of different winds ; a whiri- 

324. What is said of sudden and violent gusts, such as tempests and 
Hurricanes I 325. What are the principal phenomena of a Tornado? 
326. How is the phenomenon, called a Whirlwind, produced 7 



ICO PNEUMATICS. 

wind and water-spout, according to the idea of Dr. Franklin, arise 
from the same cause ; the only difference being that as the whirl- 
wind is formed at land, it is composed principally of air ; whereas, 
a water-spout being formed at sea, is, therefore, composed of water. 

Winds are violent or gentle, in proportion to the rarefaction or 
disturbance which has taken place in the atmosphere. To ascer- 
tain the velocity of the wind is an experiment which has attracted 
the attention of many eminent persons. Various means have been 
devised for this purpose, but the principal methods hitherto em- 
ployed are, by observing the space passed over by a cloud, or any 
light substance, and the corresponding times ; which being com- 
pared with an anemometer, the velocity of the wind may, at any 
other time, be determined by means of such an instrument. From 
such experiments it appears that wind is just perceptible when it 
moves at the rate of two or three miles an hour; that it is a gentle 
pleasant wind or breeze at four or five miles an hour ; a pleasant 
brisk gale, at ten or fifteen miles ; high, at thirty or thirty-five miles ; 
a violent wind, in a great storm, at fifty or sixty miles ; and a hur- 
ricane at eighty or a hundred miles an hour, tearing up trees and 
carrying away buildings, &c. The velocity of the trade winds is 
estimated by M. De Lalande at four or five miles an hour. 

Concerning the utility of the wind as an agent in the natural 
world, many interesting remarks might here be added, had we room 
for such. Besides the application of its power to various mechanical 
purposes, the wind is necessary to purify the atmosphere ; it raises 
and transports the clouds from the sea to fertilize the land, and 
serves to convey us over the ocean, the " great highway of the 
globe." 

2. Fill a large dish with cold water ; into the middle of this put 
a water-plate, filled with warm water. The first will represent 
the ocean ; and the other an island, rarefying the air above it. 
Blow out a wax candle, and if the air be still, on applying it suc- 
cessively to every side of the dish, the smoke will be seen to move 
towards the plate. Again, if the ambient water be warmed, and 
the plate filled with cold water, let the smoking wick of the can- 
dle be held over the plate, and the contrary will happen. 

3. If a lighted candle be held near the top of a door, when partly 
open, the flame will be blown outwards ; showing that there is a 
current of air flowing out from the upper part of the room. By 
placing the candle near the bottom of the door, the direction of the 
flame will show that there is also a current of air setting into the 
room. 

4. It is well known with what force the external cold air will 
rush through every opening into a room warmed by a fire. But 

327. What is supposed to be the cause of Water-spouts 1 328. By what 
method is the velocity of the wind ascertained 1 32J. Describe the scve- 
ral varieties of winds according to their different degrees of velocity. — 
330. What experiments serve to illustrate the foregoing explanations 
relative to the phenomena of wind ? 



PNEUMATICS. 101 

the rarefied or light air, with which the room is filled, must find 
some vent, in order to make way for the heavy air which enters. 

5. Make a large fire in an open field, and as the fire increases 
and rarefies the air, leaving a vacuum that must be filled, a press- 
ure on all sides towards the fire will soon be perceived. When 
the fire increases to a large flame, the wind rises, and at first 
blows towards it from all sides, but soon it comes with greater 
force from some colder point, and increases with the heat. 

This current of air will not subside immediately after the fire 
is out, as it can not quickly gain its equilibrium. Thus, violent 
temperatures, as in storms, great rains, tempests, hurricanes, &c. 
produce a sort of crisis in the atmosphere, which occasions a 
constant temperature, and an agitation or fluctuation in the air, 
which does not subside for many days, and sometimes several 
weeks. Hence the origin of those sudden changes in the air, 
called whirlwinds, tempests, hurricanes, &c. for the change in 
some particular parts of the atmosphere causes the air to rush in 
with such force, that more than sufficient to fill the void is moved ; 
this excess of air, like waves rolling against each other, returns 
in different directions, endeavouring to retrieve an equilibrium. 

Something of this kind may be observed by suddenly raising a 
large bottle of water from a pond or cistern ; the water from all 
sides rushes in to fill the void, and when the waves meet each 
other, they fill the vacuum and strike together; the shock returns 
and gives motion to the rest of the water at a considerable dis- 
tance. And these rarefactions in the air may happen near the 
surface of the Earth, or much above it; and this is the reason that 
clouds are sometimes seen moving in different directions. 

VI. The pressure of the atmosphere varies at different alti- 
tudes. 

Exp. Put a glass tube, open at both ends, through a cork, into 
a large phial containing a small quantity of coloured water ; let 
the lower end of the tube be in tne water, and let the cork and 
tube be closely cemented to the neck of the bottle. Then blow 
through the tube, till the quantity of the air within the phial is 
so increased, that the water will rise above the neck of the phial. 
Let this phial be placed in a vessel of sand, to keep the air within 
of the same temperature ; the water will stand at different heights 
in the tube, according to the elevation of the place where it is 
placed ; whence it appears, that the pressure of the atmosphere 
varies at different altitudes. 

Corol. Hence the proportion of the specific gravity of air to 
that of water may be determined. If the difference in height of 
the two places where the above experiment is made be 54 feet, and 
that difference cause a difference of £ of an inch in the height of 

33,1. What is the reason that clouds are sometimes seen moving in 
different directions,'? 332. What simple experiment proves that the 
pressure of the atmosphere varies at different altitudes 1 



102 PNEUMATICS. 

the water; it follows, that a column of water off of an inch, or 
one sixteenth of a foot, is equiponderant to a column of air of 54 
feet haying the same base ; therefore, the gravity of air to that of 
water, is 54 to one-sixteenth, or 864 to 1. In ascending the moun- 
tain of Snowden in Wales, which is 3720 feet perpendicular 
height, it was found, that the mercury in the barometer sunk three 
inches and eight-tenths. 

Obs. Experience shows that the atmosphere, or the air which 
surrounds the Earth, is of different densities at different distances 
from the surface of the Earth. This is obvious from the conside- 
ration that those portions of the air which are nearest to the 
Earth's surface, are pressed down by the weight of the superin- 
cumbent parts of the atmosphere, and sustain the whole pressure 
of the atmosphere, while the higher strata bear only apart of the 
pressure. This pressure decreases upwards, and consequently, 
the air becomes proportionally rare, it being capable of expanding 
by its own elastic force ; for the atmospheric column diminishes 
in length as the distance from the Earth's surface increases. 

From experiments, assisted by calculation, a very remarkable 
fact has been ascertained, which is, — If the altitudes above the sur- 
face of the Earth be taken in arithmetical progression, the density 
of the air at those cdtitudes will be in geometrical progression de- 
creasing. Thus, for instance, if at a certain altitude the air is 
half as dense as it is immediately on the surface of the Earth; 
then, at twice that altitude, the air will be four times less dense 
than upon the surface of the Earth ; at three times that altitude, 
it will be eight times less dense; and so on. For example, at the 
height of seven miles, the density of the air is one-fourth ; at the 
height of fourteen miles, one-sixteenth; at the height of twenty- 
one miles, one sixty-fourth ; and so on. 

PNEUMATIC INSTRUMENTS. 

CXXX. The Syringe, or common water- 
squirt, is a hollow tube with a small orifice at 
one end ; at the other end is inserted a solid cyl- 
inder, so exactly fitted to the tube that no air can 
pass along its sides, and a fixed handle to the 
solid cylinder. If that end of this instrument, 
which has the smaller orifice, be drawn back, a 
vacuum will be produced within the syringe : 

333. What is said concerning the density of the atmosphere at differ- 
ent distances from the Earth's surface? 334. What law has been as- 
certained with respect to the density of the atmosphere at different alti- 
tudes? 335. What illustration is given of this law? 336. What is a 
tS'jringe 7 337. W T hat is its principle of action ? 



PNEUMATICS. 103 

and the pressure of the atmosphere on the sur- 
face of the water, meeting with no opposite pres- 
sure, will force the water into the tube, from 
which it may be forcibly expelled by pushing 
down the piston. 

CXXXI. The Condenser is used to force air 
into any vessel ; it is a syringe, having a solid 
piston, and a valve in the lower part of its barrel, 
which opens downwards. By thrusting down 
the piston, the air is forced through the valve, 
which is afterwards held close by the elasticity 
of the condensed air. "When the piston is lifted 
up, a vacuum is produced, till it is raised above a 
small hole in the barrel, when the air rushes in 3 
and is again discharged through the valve. 

CXXXII. The Air-Gun, is an instrument, 
in the form of a gun, by which a quantity of 
condensed air is suddenly set free, and drives a 
ball through the barrel with great force. 

CXXXIII. The Barometer is a very useful 
instrument for measuring the weight or pressure 
of the atmosphere: for determining the height of 
towers, mountains; the depths of hollows, pits, 
&c, and for ascertaining the variations in the 
state of the air, and predicting the changes in 
the weather. 

Obs. 1. This instrument consists of a glass tube, about an inch 
in diameter, and 32 or 33 inches long, filled with mercury, having 
its upper end closed, and the lower end immersed in a basin of 
the same fluid, and the whole neatly set in an upright case. As 
the external air presses with a greater or less weight on the mer- 
cury in the basin, it causes the column of mercury to be more or 

33?. What is a Condenser 1 339. Describe its construction, and the 
principle upon which it acts. 340. How is the Air-gun constructed 1 
3-11. What is the use of the Barometer 7 442. How is a barometer con- 
structed % 



104 



PNEUMATICS. 






Jess elevated in the tube, and by measuring the height of this co- 
lumn by means of a scale, the degree of pressure or weight of the 
air may be accurately ascertained. The mean height of the mer- 
cury in a barometer at the level of the sea, is about 30 inches ) and, 
it is never below 23, nor above 31 inches. 

Exp. 1. If a glass tube about 32 or 33 inches long, hermetically 
sealed at one end, be filled with mercury, and then inverted into 
a basin of the same fluid, the mercury in the tube will stand at an 
altitude above the surface of that in the basin between 28 and 31 
inches. A tube thus filled, and graduated from 28 to 31 inches, is 
called a barometer. Hence, as a cubic inch of water weighs 
252.54 grains, Troy, a cubic inch of air weighs.305 grains, and sup- 
posing mercury to be about 13$ times heavier than water, the spe- 
cific gravity of air is to that of mercury as 1 to 828X13£, or as 1 to 
11178. 

2. Now the mercury in the barometer tube will subside, till the 
column be equivalent to the weight of the external air upon the 
surface of the mercury in the basin, and is therefore a true crite- 
rion to measure that weight, and chiefly directed to that purpose, 
in order to foretel the changes in the weather. 

Obs. 2. If each inch in the scale of variation be divided into 
ten equal parts, marked 1, 2, 3, increasing upwards, and a vernier 
whose length is 1 tV of an inch, be likewise divided into ten 
equal parts, increasing downwards, and so placed as to slide along 
the graduated scale of the barometer, the altitude of the mercury in 
the tube, above the surface of that in the basin, may be found, in 
inches and hundredth parts of an inch, by this vernier. If the 
surface of the mercury in the tube does not exactly coincide with a 
division in the scale of variation, place the index of the vernier 
even with this surface, and observing where a division of the ver- 
nier exactly coincides with one of the scale, the figure in the ver- 
nier will show what hundredth parts of an inch are to be added to 
the tenths immediately below the index. 

3. If the atmosphere were homogeneous, its altitude would be 
easily found. For when the mercury stood at 30 inches, the den- 
sity of the air being to that of mercury as 1 to 11178; conse- 
quently, the altitude of a homogeneous atmosphere would be equal 
to 1117SX30=5.29 miles. 

4. The barometer has been applied to the measuring of the 
height of towers, mountains, &c. Since 1 1 178 inches of air, near 
the surface of the Earth, are equal to one inch of mercury ; 1118 
inches, or about 93 feet of air, must correspond to one-tenth of an 
inch of mercury. Therefore, if a barometer be carried up anv 
great eminence* the mercury will descend one-tenth of an incn 
for every 93 feet that the barometer ascends. 

343. At what height will the weight of the atmosphere sustain the co- 
lumn of mercury 1 344. What is the rule for measuring altitudes with* 
Hit barometer 1 



PNEUMATICS. 105 

CXXXIV. The Thermometer is an instru- 
ment designed for the purpose of measuring the 
temperature of the air, and of other bodies ; and 
usually consists of a long slender glass tube, con- 
taining spirits of wine, mercury, &c, which flu- 
ids are found to swell and occupy different por- 
tions of the tube in different temperatures, 

Obs. 1 . The glass ball, called the bulb, at the bottom of the tube, 
is filled with a fluid which is most commonly mercury. — 
The top of the tube is closed, and is so contrived as to be perfectly 
free from air. An increase of heat expands the mercury, and by 
that means causes it to rise in the tube ; cold, on the contrary, 
contracts or diminishes the bulk of the mercury, and it then sinks 
lower in the tube, consequently the temperature of the contiguous 
bodies, is known by the rise and fall of the mercury in the tube, 
and is measured by a scale. 

2. In dividing the scale attached to a thermometer, the tempe- 
rature of ice and that of boiling water, supposed to be invariable, 
are assumed as standard points, with which to begin the scale. 

3. There are many kinds of thermometers, but that most com- 
monly used is denominated Fahrenheit's from the name of the 
invented The only material difference, however, in thermome- 
ters consists fii the graduation of the scale. 

According to the scale of Fahrenheit, which is the most ill con- 
trived of all, the height of the mercury, when the thermometer is 
immersed in pounded ice or freezing water, is marked 32 degrees, 
and its height in boiling water is marked 212 degrees. The space 
between these two points is divided into 180 equal parts, and num- 
bered accordingly. The divisions are also extended both above 
and below these points, as far as convenient. If the divisions ex- 
tend below the cypher 0, called also zero, they are numbered 
downwards, thus ; 1, — 2, &c. 

4. The scale on Reaumur's thermometer, which has been princi- 
pally used on the Continent, begins at the freezing point and pro- 
ceeds both ways, from or zero. From the freezing to the boiling 
point, are SO degrees. 

5. The scale of Fahrenheit, it must be confessed, is as badly 
contrived as can well be imagined ; but, a far more rational and 
consistent division of the thermometrical scale was proposed by 
Celsius and Christin, and has been generally adopted in France. 

345. What kind of instrument is the Thermometer 7 346. On what 
principle does a thermometer indicate the temperature of the air, and of 
other bodies 1 347. What causes the rise and fall of the fluid in a ther- 
mometer 1 348. What is the construction of the different kinds of ther- 
mometers 1 



106 PNEUMATICS. 

This kind, which is known as the Centigrade thermometer, has 
the freezing point marked 0, and the boiling point 100. 

6. As it' may be often found convenient to be able to reduce 
the degrees of one kind of thermometer to the corresponding ones 
of an other, the following directions are given for this purpose. 

I. To reduce the degrees of Fahrenheit into those of the Centi- 
grade scale. 

Rule. — From the given degree deduct 32°, multiply the 
remainder by 5, and divide the product by 9. 

Exam. To find what degree of the Centigrade corresponds to 
the 95th degree of Fahrenheit ;— Thus, 95 — 32=63, 63X5=315, 
and G15-r-9=35=the degree required. 

ii. To convert the degrees of the Centigrade to those of Fah- 
renheit. 

Rule. Multiply the given degree by 9, divide the product by 5,. 
and add 3 2° to the quotient. 

Exam. To reduce the 90th degree of Fahrenheit to the corres- 
ponding degree of the Centigrade; Thus, 90X9=810, 810-5-5== 
162, and 162-|-32=194=the degree required. 

in. To convert the degrees of Fahrenheit into those of Reau 
mur; — (F — 32) -$- = R — thus, the 167th degree of Fahrenhei 
answers to the 60lh of Reaumur. 

iv. To convert the degrees of Reaumur to those of Fahrenheit ; 
RX9-i-4-(-32=F. Thus, the 40th degree of Reaumur answers 
to the 122d degree of Fahrenheit. 

7. Mr. Wedge wood contrived a thermometer for measuring 
higher degrees of heat, by means of a property of argillaceous 
bodies — the diminution of their bulk by fire. This diminution 
commences in a dull red heat, and proceeds regularly as the heat 
increases, till the clay becomes vitrified. 

Each degree of Wedge wood's thermometer answers to 130 de- 
grees of Fahrenheit ; and the scale begins from 1077 of Fah- 
renheit. 

TABLE 

Of the effects of heat, fyc. t as indicated by a Thermometer. 



NAMES OF SUESTANCES. 




FAHR. 


CENTT. 


Strongest Nitric Acid, 


freezes. 


—55°' 


—48° 


Ether, and liquid Ammonia 


f< 


—46 


—43 


Mercury, 


(( 


—39 


—39 


Brandy, . . - . 


(C 


—7 


—22 


Strongest sulphuric acid, 


(( 


+ 1 


—17 



319. Describe the graduation of the scale on Fahrenheit's thermome- 
ter ; on Reaumur's ; and on the Centigrade thermometer. 3.50. What 
are the freezing and boiling points 1 351. What is the rule for reducing 
the degrees of the Fahrenheit thermometer, into those of the Centigrade 
scale, and conversely 7 352. What is the rule in regard to Fahrenheit and. 
Rcaumurs thermometers 353. What is the principle of Wedgewood's 
thermometer, or Pyrometer, as it is more usually denominated 1 



PNEUMATICS. 

Oil of Turpentine, 

Strong wines, . . . 

Human blood, . ... 

Vinegar, and Urine, ... 

Milk, . , • • J 

Water, *. . 

Olive oil,' .... 

Equal parts of Sulphur and Phosphorus 

Butter, .... 

Lard, . 

Phosphorus, 

Spermaceti, .... 

Tallow, .... 

Beeswax, .... 

Bleached wax, 

Sulphur, .... 

Camphor, .... 

Tin,* . 

Bismuth,* 

Lead,* * 

Zinc, • 

Antimony, . 



melts, 



Brass, . . . 

Copper, 

Silver, .... 

Gold, .... 

Cobalt, .... 

Nickel, 

Soft nails, 

Iron, . . • 

Manganese, and cast Iron, 

Platinum, Tungsten, Uranium, &c, 

Ether, . 

Liquid Ammonia, . * 

Camphor sublimes, 

Sulphur evaporates, 

Alcohol, . . 

Water, and essential oils, 

Cow's milk, 

Nitrous Acid, 

Nitric Acid, 

Metallic Arsenic sublimes, 

Phosphorus, 

Oil of TurjDentine, 

Sulphur, . 

Sulphuric acid, 



melts, 



boils. 



boils, 



boils, 



21 
27 
25 
32 
130 
150 
154 
153 
160 
170 



16 

20 
25 

28 

30 

32 

38 

40 

84 

87 

90 

112 

120 

142 

154 

220 

303 

442 

476 

612 

680 

809 

3309 

458? 

4717 

5237 

17977 

20577 

21097 

21637 

21877 

23177 

98 

140 

14 

170 

176 

212 

213 

242 

248 

540 

554 

560 

570 

590 



107 

—9 

—7 

—4 

—2 

—1 



+3 

4 

29 

30 

32 

44 

49 i 

61 

68 

104 

151 

228 

247 

322 

360 

432 

2098 

2531 

2603 

2392 

9969 

11414 

11703 

12003 

12136 

12858 

37 

60 

63 

77 

80 

100 

101 

117 

120 

232 

290 

293 

299 

310 



* A mixture of these three metals in the proportion of 3, 8, and 5, will- 
melt in a degree of heat below that of boiling water. 



108 PNEUMATICS. 

Linseed oil, . . . ' 

Mercury, . . . . ' 

Miscellaneous effects of heat ) fyc. 
Greatest cold produced by artificial means, 
Cold observed at Petersburg!!, Dec. 1772, 
Cold produced at Hudson's Bay, by a m 

sulphuric acid and snow, 
Natural cold at Hudson's Buy, 
Equal parts of snow and common salt, 
Phosphorus burns slowly, 
Constant heat in the vaults below the observatory at 

Paris, . . 
Vinous fermentation begins, 
Mean temperature of the surface of the globe, 
Acetous fermentation begins, 
Animal temperature, 92° to 99. 
Heat in the interior of a human body, 
Fever heat, 103 to 112, 
Heat at which chickens are hatched. 
Heat observed in Senegal, 
Phosphorus burns vividly, 
Sulphur burns slowly, 
Lowest ignition of iron in the dark, 
Hydrogen in flames, 
Charcoal burns, 
Iron red in twilight, 

Iron red in daylight, 

Diamond burns, 

Working heat of plate glass, 

Heat of flint glass furnaces, 

Least welding heat of iron, 

Stone ware fused, 

Greatest heat of a flint glass furnace, 

A smith's forge, . 

Hessian crucible fused, 

Greatest heat observed, . 

Extremity of Wedgewood's scale, 

CXXXV. The Hygrometer is an instru- 
ment for measuring the degrees of moisture in 
the air ; of which there are various kinds ; for 
whatever contracts and expands by the moisture 
and dryness of the atmosphere, is capable of be- 







600 


316 




660 


349 




—90 


—68 


, 


—80 


—62 


:ture of 






. 


—69 


—56 


. 


—50 


—46 


. 


zeroO 


—18 


atory at 


+43 


+6 


54 


12 


. 


59 


15 


; > 


60 


16 


. 


77 


25 


. 


96 


36 


. 


102 


39 


. 


107 


42 


_ 


110 


43 


, 


116 


47 


, 


122 


50 


? 


303 


150 




635 


335 


. 


800 


427 


.•< 


802 


428 


M 


1050 


566 




1 


1207 


653 




6 


2897 


1592 




57 


8487 


4697 




70 


10177 


5636 




90 


12777 


7081 


. 


102 


14337 


7947 




114 


15897 


8814 




125 


17327 


9608 




150 


20577 


11414 




185 


25127 


13942 




240 


32277 


17914 



35-1. What is a Hygrometer 7 



ACOUSTICS. 109 

ing formed into a hygrometer. Such ar.e most 
kinds of wood ; catgut ; twisted cord ; the beard 
of wild oats ; the weather house, &c. 

Obs. A hygrometer may be made by stretching a catgut, or 
common cord, along a wall, passing it over a pulley ; fixing it at 
one end, and to the other hanging a weight, carrying a small in- 
dex. Against the same wall, fit a metal plate, divided into any 
number of equal parts, and the hygrometer is complete. 

ACOUSTICS. 

CXXXVI. Acoustics is the science which 
treats of the nature, phenomena, and laws of the 
sense of sound. It extends to the theory of mu- 
sical concord and harmony, and is, therefore, a 
valuable and interesting science. 

CXXXVII. Sound is considered as arising 
from vibrations in the air, communicated to it by 
vibrations of the sounding body, acting in pulsa- 
tions or concentric waves, like the surface of 
water when a stone is thrown into it. 

Obs. If when a piece of artillery is fired at a distance, some 
dust floating in the air, or a cobweb be closely inspected, it will 
be seen to be agitated at the instant when the report is heard. — 
This proves that the vibrations of the air move with the same 
velocity that sound does, and that it is by means of these vibra- 
tions striking on the tympanum or drum of the ear, that sounds 
are conveyed. 

Each vibration of the particles of the sounding body is succes- 
sively propagated to the particles of the air, till it reaches those 
which are contiguous to the tympanum of the ear, a fine mem- 
brane distended across the ear; and these particles, in performing 
their vibrations, impinge upon the tympanum, which agitates the 
air included within it, which being put into a like tremulous mo- 
tion, affects the auditory nerves ; and thus excites in the mind the 
sensation or idea of what we call sound. 

CXXXVIII. A sonorous body, whilst sound- 

355. Of what substances may a Hygrometer be constructed ? 

356. What is sound ; how does it aiise ; and how is it communicated'? 
357. What is proved of these vibrations of the air % 35^. Is air neces- 
sary to the production of sound'? 359. How is it proved that sound 
arises from the vibrations of the air"? 

10 



110 ACOUSTICS. 

ing, is unquestionably in a state of vibration, 
and the air, by similar vibrations, communicates 
and propagates these vibrations. 

Obs. 1. That sensation which is produced or conveyed into the 
ear is called sound, though, in truth, until it reaches and affects 
the perceptive part, it is nothing but motion. 

2. The motion which produces in us the perception of sound, is 
a vibration of the air, caused by an exceedingly short but quick, 
tremulous motion of the body, from which it is propagated ; and 
therefore we consider and denominate them as bodies sounding. 

3. That sound is the effect of such a short, brisk, vibratory mo- 
tion of the bodies from which it is propagated, may be known 
from what is observed and felt in the strings of instruments, and 
the trembling of bells, as long as we perceive any sound come 
from them ; for as soon as that vibration is stopped, or ceases in 
them, the perception ceases also. 

CXXXIX. The principal causes of the va- 
riety of sounds, are ; 

First, the greater or less frequency of the 
vibration. 

Secondly, The quantity or force of the vibra- 
ting materials. 

And, Thirdly, The greater or less simplicity 
of the sounds. 

Hence arise the height, the strength, and the 
modification of sounds. 

Obs. When sounds are equally acute, they are said to have the 
same pitch ; but when they differ in acuteness, that sound which 
is shriller is said to be acute, or have a higher pitch ; and that 
which is less shrill, is said to be graver, or to have a lower pitch, 
or a deeper tone. A difference in pitch, forms the chief character 
by which musical sounds are distinguished from each other, and is 
the foundation of their use in music. 

CXL. The vibrations of a sounding body con- 
tinue for a longer or shorter time, according as 
the body is more or less elastic, or as it is thicker 
or thinner. 

360. What are the principal causes of the varieties of sounds 1 361. 
What is said concerning the pitch of a sound 1 



ACOUSTICS. Ill 




H »the 



Exam. When a string of uni- 
form shape and quality, is stretched 
between two steady pins, and fixed to 
* them, as a b, Fig". 43. if it be drawn 
out of its natural or quiescent posi- 
tion a b, into the situation a c b, and 

then be let go, it will in consequence of its elasticity, not only 
come back to its position a b, but it will go beyond it "to the situ- 
ation of a db, or nearly as far from a b. as a c b was on the other 
side. All the motion one way. is called one vibration ; after this, 
the string will go again nearly as far as c, making a second vibra- 
tion ; then nearly as far as d, making a third vibration, and so on ; 
diminishing the extent of its vibrations gradually, until it settles 
v n in its original position a b. 

I (lbs. 1. During the whole of these vibrations, the string will 
foxibly act on the air. and produce corresponding vibrations in it, 
w iLh, reaching and entering the ear, produce on the nerves there- 
in tie sense of sound. 

'.'. The following experiment indicates a curious accordance of 
vjb. Etion, and proves that the air re-acts in the exact law of the 
orig.'nal vibration. 
Fig. 44. 

Exp. Divide a string as a b, Fig. 44, into 

three equal parts, ab, be. cd, by placing dots 

fL f> C 3- at c and b; place a bridge like a violin 

I 1 \ \ bridge, at b, also place light bodies, such as 

small bits of paper, at c, and other places 
of the part b d. Then draw a violin bow over the part a b ; we 
shall find that all the bits of paper will be thrown off from the 
part b d, excepting the one at c; showing that the point c remains 
at rest, whilst the remainder of the string is vibrating, just as 
though c also had a stop, as at b. 

CXLI. Sounds in general are conveyed to the 
ear by means of the air ; but water is also a good 
conductor of sound ; as are timber and flannel. 

Exp. 1. A bell rung under water, returns a tone as distinct as 
if rung in air. 

2. If we stop one ear with a finger, and the other by pressing it 
eiose to a long stick or piece of deal board, and a watch be held at 
the other end of the wood, the ticking will be heard, be the stick 
or board ever so long. 

362. On what does the continuance of the vibrations depend, for a 
longer or shorter lime 1 363. How may this be illustrated 1 364. What 
other substances, besides air. convey the vibratory motion of sonorous 
bodies 1 365. What illustrations are given of the 'power of liquids and 
solids to conduct sounds 1 



112 ACOUSTICS. 

3. If we tie a poker or any piece of metal on to the middle of a 
strip of flannel, about two or three feet long, and then press with 
the thumbs or fingers the ends of the flannel in the ears, while we 
swing the poker against an iron or steel fender, we shall hear a 
sound like that of a very heavy church bell. 

4. If two persons stop their ears, they may converse with each 
other, by holding the two ends of a stick between their teeth, or, 
only resting the ends of the stick against their teeth. The same 
may be done by a series of sticks, with the ends touching each 
other. The same effect is also produced if the end of the stick 
rest on the throat, or breast, or if one end of it touch a vessel into 
which the other speaks. In the last instance the sound is most 
distinct if the vessel is capable of a tremulous motion, as one of 
glass, bell-metal, or copper. 

Sound may also be conveyed from one person to an other by a 
string stretched between their teeth. 

CXLII. Sound moves at the rate of 1142 feet 
in a second, or about thirteen miles in a minute * 
This is the case with all kinds of sounds, when 
conveyed by means of air ; the softest whisper 
flies as fast as the loudest thunder. 

Obs. The velocity of sound has been applied to the measure- 
ment of distances. 

1. A ship at sea in distress fires a gun, the light of which is 
seen on shore 20 seconds before the report is heard, therefore it is 
known to be at the distance of 20 times 1142 feet, or little more 
than 4£ miles. 

2. I see a vivid flash of lightning, and if in three seconds I hear 
a tremendous clap of thunder, I instantly know that the thunder 
cloud is only two-thirds of a mile distant, I should therefore retire 
instantly from any exposed situation. 

366. What is the estimated velocity of sounds through the air 1 367. To 
what practical purposes can we apply the uniform velocity of sound 1 



* The velocity of sound, as here stated, is in accordance with the de- 
termination given by Dr. Derham, who is considered as having made the 
greatest number of accurate and more diversified experiments. There 
is considerable difference observable in the results obtained by different 
philosophers. Cassini and the other French Academicians, ascertained 
the velccity of sound at 1107 feet per second ; the members of the Floren- 
tine Academy at 1143 ; those of the Royal Academy of Sciences. 1172 Pa- 
risian feet; Gassendus computed it at 1473 ; Mersenne, at 1471; Duha- 
mel, at 1 33 3 ; Newton, at 968; according to some very accurate experi- 
ments, 1130; and Derham, at 1142, which corresponds with Flamstead 
and Dr. Halley's determination. The Board of Longitude'renewed the 
experiments in the month of June, a year or two since with all possible 
precision, when it was found that the velocity of sound in the air, at the 
temperature of 55 Q Fahr. differs very little from 1044 feet per second. 



ACOUSTICS. 113 

3. The pulse of a healthy person beats about 76 times in a 
minute ; if, therefore, between the flash of lightning and the thun- 
der, I can feel 1, 2, 3, 4, &c. beats of my pulse, I know the cloud 
is 900, 1800, 2700, 36,000 feet from me, 

CXLIII. Sounds are capable of being reflected 
by hard bodies, or plane surfaces ; and this reflec- 
tion produces the well known phenomenon, call- 
ed an Echo; and other similar phenomena. 
Sound, like light, after it has been reflected from 
several places may be collected into one point as 
a focus, where it will be more audible than in 
any other part ; and on this principle whisper- 
ing galleries are constructed. 

Obs. In the reflection of sound, as -well as of light, the angle of 
reflection is equal to the angle of incidence. By the same law, 
therefore, sound may be collected into a focus. 

Exp. i. If the pulses cf air conveying sound be suffered to im- 
pinge on a concave surface, the reflected vibrations are converged 
into a focus. 

2. The same effect is produced whenever a number of plane 
surfaces are so situated that the reflected sounds meet, and cross 
each other at a certain point. If the ear be placed at this point, 
the sound will be audible in proportion to the number of surfaces 
so placed. The famous whispering gallery at St. Paul's is con- 
structed on this principle. 

CXLIV. Speaking Trumpets, and those 
which are made to assist the faculty of hearing 
in deaf persons, depend on the reflection of sound 
from the sides of the trumpet, and also by its 
being confined and prevented from spreading in 
every direction. 

Obs. I. A speaking trumpet, to have its full effect, must be di- 
rected in a line towards the hearer ; the report of a gun or cannon 
is much louder when fired towards a person, than one placed in a 
contrary direction. 

«^ 

363. What is said concerning the reflection of sound 1 369. How is 
the sound of an Echo produced ? 370. On what principle are Whisper- 
ing Galleries constructed % 371. What is observed of plane and concave 
surfaces in converging sound into a focus ? 372. On what principle are 
Speaking Trumpets constructed 1 

10* 



114 ACOUSTICS. 

2. The human voice is produced by the expulsion of air from 
the longs, and by the vibrations excited in that air, by a very small 
membrane called the glottis, in its passage through the trachea or 
windpipe ; and by the subtile modification of the mouth, tongue, 
and lips. 

3. Singing is performed by a very delicate enlargement or con- 
traction of the glottis, aided likewise by the mouth and tongue for 
articulation. 

4. In stringed instruments the air is struck by the string, and the 
vibrations of the air produce corresponding sounds in the ear ; but 
in pipes, the air is forced against tne sides by the breath, and its 
vibrations or tones produced by the reaction of the sides. 

CXLV. An echo is the reflection of sound 
striking against a surface fitted for the purpose, 
as the side of a house, a brick wall, hill, &c. and 
returning to the ear at distinct intervals of time. 

Obs. 1. If a person stand about 65 or 70 feet from such a surface, 
and perpendicular to it, and speak, the sound will strike against 
the wall and be reflected, so that he will hear it as it goes to the 
wall, and again on its return. 

2. If a bell, situated in the same way be struck, and an observer 
stand between the bell and the reflecting surface, he will hear the 
sound going to the wall, and again on its return. 

3. If the sound strike the wall obliquely, it will go off oblique- 
ly, so that a person who stands in a direct line between the beli 
and the wall, will not hear the echo. 

CXLVI. Concord is any succession of sounds 
that excite in the ear certain agreeable sensations. 
Sound is therefore the subject matter of musical 
science. Harmony is the coincidence of two or 
more sounds, which by their union afford to the 
mind pleasure and delight. 

Obs. 1. Concord arises from the agreement of the vibrations of 
two sonorous bodies ; so that some of the vibrations of each strike 
upon the ear at the same instant. 

Thus if the vibrations of two strings are performed in equal 
times, the same tone is produced by both, and they arc said to be 

373. How is the human voice produced? 374. What parts are concern- 
ed in the modulation of the voice in singing? 375. How does the produc- 
tion of sound by pipes differ from that by strings? 370. What is an 
Echol '611 . How is an echo accounted for? 378. What is observed con- 
cerning the reflection of sound and light following the same laws ? 379. 
What is Concord ? 



ACOUSTICS. 115 

in Unison. If the vibrations strike the ear at different times there 
is no unison, and consequently a discord is produced. 

Obs. 2. Concord is not confined to unison. In this case no va- 
riety of tones would be produced. It is the effect of agreement 
between vibrations. 

Illus. If the vibrations of one string are double those o£ an other 
in the same time, the second vibration of the latter will strike 
upon the ear at the same instant with the first vibration of the 
former ; this makes the concord of an octave. 

CXLVII. Two strings of equal length, ten- 
sion, and thickness, by performing their vibra- 
tions together, will sound the same note, or be in 
unison. Two pipes of the same length and dia- 
meter, will agree in the same manner. — Large 
instruments and long strings produce grave or 
deep tones ; small instruments and short strings 
produce acute and high tones. 

Obs. 1. In the case of the strings, the air is struck by the body, 
and the sound is excited by the vibrations ; in that of the pipes the 
body is struck by the air, but as action and reaction are equal, the 
effect is the same. 

2. Let a musical string of any length be divided into two equal 
parts by a bridge in the middle ; and the sound of each half is 
eight notes, or an octave, higher than the tone of the whole string. 

Organ pipes produce grave or acute tones in proportion to their 
length and size. It is the shortest string of a harpsichord which 
yields the highest notes. 

CXLVIII. Sounds maybe conveyed to a much 
greater distance through a continuous tube, than 
through the open air. 

Illus. Pipes are used in some taverns, running from one room 
to an other to convey orders to the servants. 

Dr. Herschel employed a similar tube attached to his forty feet 
telescope for communicating his observations to an assistant who 
satin a small house near the instrument; and thus under cover 

380. From what does concord arise? 381. How are harsh jarring 
sounds or Discords produced'? 382. Under what circumstances will two 
strings or two pipes be in Unison! 3S3. How is an Octave concord 

Eroduced"? 384 In what manner may sounds be conveyed to a greatei 
istance than through the open air 1 335. What is remarked concerning 
Dr. Herschel's method of communicating his observations to an assist- 
ant? 



116 OPTICS. 

noted them down, and the particular time at which they were 
made. 

Obs. The tubes used to convey sounds are called Acoustic 
tubes. 

It is by means of such tubes that the deception of what is 
called the Invisible Lady is carried on. In this exhibition, a 
square railing of wood is fixed in the middle of the room, and 
within the railing, a globe is fixed, having four trumpets inserted 
into it, one opposite to each side of the railing. The spectators 
ask a question by speaking into one of the trumpets, and then on 
holding the ear to the same trumpet, they receive the answer. 
This deception is performed by conveying the sound by tubes, 
which are carried from one room to an other under the flooring, 
and within the bar of the railing to a small aperture opposite to 
the mouth of the trumpet. When the question is asked, it is con- 
veyed by this pipe to a person placed in the next room, and the 
reply is conveyed back to the trumpet by the same tube. 

OPTICS ;* 

OR, 

THE LAWS OF LIGHT AND VISION. 

CXLIX. That science which treats of the na- 
ture and properties of Light, and of the phe- 
nomena depending on those properties, is called 
Optics. 

CL. Light is that principle which, proceeding 
from any body to the eye, produces the sensation 
of vision, or the perception of seeing, that is, 
renders objects visible. 

CLI. Light, according to some philosophers, 
consists, either of small particles emanating from 
a luminous body, or of vibrations excited by 

386. How is the deception of the Invisible Lady managed 387. What 
is the name of that science which treats of vision, or of the nature and 
properties of light ? 388. What is Light 7 889. What consideration 
demonstrates the materiality of light. 



"Theterm Optics, which comprehends whatever belongs to light and 
vision, is derived from a Greek word, which signifies seeing. That 
branch of the science of Optics which treats of reflected light, is called 
Catoptrics ; and that which treats of refracted light, is called Dioptrics. 
Chromatics is that branch of Optics, which relates to the phenomena of 
colours. 



OPTICS. 117 

combustion in a universal medium, which/pro- 
ceeding to the eye, produce the perception of 
vision. 

Obs. 1. It is evident we can not see any object by willing or 
wishing to see it ; but that something must pass from the object 
to the eye, because no effect can take place without a proximate 
cause. 

2. Sir Isaac Newton supposed rays of light to consist of exceed- 
ingly small particles, infinitely smaller than sand, moving from 
luminous bodies ; but later writers suppose them to consist of the 
undulations of an elastic medium, which fills all space, and which 
produces the sensation of light to the eve, just as the vibrations of 
air produce the sensation of sound to the ear. We shall not pre- 
fer either hypothesis in these pages, because either of them will 
account equally for all the phenomena of light.* 

3. Light is universally expanded through space, and exerts 
peculiar actions on the various bodies., to which its influence 
extends. In some respects, light is obedient to the laws 
which govern bodies ; in others, it appears to be independent 
of them. Thus, though its course is guided by the laws of mo- 
tion, it does not appear to be influenced by those of gravity. — It 
has never been discovered to have weight, though a variety of in- 
teresting experiments have been made with a view of ascertaining 
that point. These experiments, however, serve to show that light, 
in its progress through space, has a sensible momentum. 

OLII. Bodies in respect of light, may be di- 
vided into three classes, — Luminous^ Opaque, 
and Transparent. 

Those bodies which shine by their own light 

390. What was Sir Isaac Newton's opinion concerning the nature of 
light'? 391. What is said of the theory of Descartes and Huygens 1 392. 
In what manner is light produced from luminous bodies I ~ 393. How 
may bodies be divided in respect of light 7 394. What is a Luminous 
body ] 



"In our opinion, this ambiguous theory of light is quite exceptionable ; 
for, by the most enlightened philosophers, light is regarded as a sub- 
stance consisting of a vast number of exceedingly "small particles, 
which are actually projected from luminous bodies, and probably never 
return to the body "from which they are emitted. 

The doctrine "that the rays of light consist of the undulations of an 
elastic medium which fills all space, and which produces the sensation 
of light to the eye just as the vibrations of air produce the sensation of 
sound to the ear. was supported by many eminent philosophers, among 
whom we may particularly mention the celebrated Descartes and Chris* 
tian Huygens. 



118 orTics. 

or emit rays of light, are called Luminous ; such 
as the San and Fixed Stars. 

Bodies which are not luminous, and which do 
not admit the rays of light to pass through them, 
are denominated Opaque bodies, as iron, earth, 
stone, wood, &c. 

Bodies which admit the rays of light to pass 
through them, are called Transparent bodies, 
such as air, glass, and water. 

Obs. 1. Light may be considered in two respects ; as it radiates 
from luminous bodies directly to our eyes, in consequence of which 
we see luminous bodies themselves, as the Sun, flame, &c, — and, 
as it is reflected from opaque bodies ; — thus we see a man or a 
picture, &c. 

2. The rays of light themselves are not seen, but by them the 
bodies from which they originally emanated ; as the Sun, a fixed 
star, or the bodies from which they are reflected ; as a horse, a 
house, or a tree. When the Moon shines, we do not see the rays 
which proceed from the Sun to the Moon, but by them we see the 
Moon from which they are reflected. 

CLIII. A body which is transparent, or which 
affords a ready passage for the rays of light, is 
called a Transparent Medium, or simply a Me- 
dium. Thus, air, water, and glass, are media, 
and the rays of light which pass through them 
are said to be transmitted by them. 

CLIV. A Ray of Light is any exceedingly 
small portion of light, as it comes from a lufni- 
nous body. 

A collection of rays flowing in parallel lines, 
is called a Beam of Light. 

CLV. Rays of light which coming from a point, 

395. What class of bodies is denominated Opaque 7 396. What are 
Transparent bodies? 397. In what two points of view may light be 
considered I 398. Are the rays of light visible to us? 399. What are 
transparent bodies sometimes called ? 400. What is a Ray of Lightf 
401. What is a Beam of Light 1 



OPTICS. 119 

and continually separating as they proceed, are 
called Diverging Rays. 

Rays which tend to a common point, are called 
Converging Rays. 

Rays of light are Parallel, when the lines in 
which they move are parallel. 

Obs. The divergency or convergencjr of rays is measured by 
the angle contained between the lines which the rays describe. 

CLVL A body of rays, either diverging or 
converging, is called a Pencil of Rays. 

CLVII. The point from which diverging rays 
proceed, is called the Radiant Point ; that to 
which converging rays are directed, is called the 
Focus. A ray of light, bent from a straight course 
in the same medium, is said to be Inflected ; 
when turned back on the surface of a body, it is 
said to be Reflected ; and, when turned out of its 
course in passing out of one medium into an 
other, it is said to be Refracted. 

CLVIII. Every visible body emits particles of 
light, or reflects vibrations from its surface in all 
directions, which, passing without any obstruc- 
tion, move within the same medium in straight 
lines. 

Obs. Wherever a spectator is placed, in respect to the luminous 
body, every point of that part of the surface which is turned to- 
wards him is visible to him ; the particles or vibrations of light 
are, therefore, emitted in all directions, and those rays only are 
intercepted in their passage by an interposed object, which would 
be intercepted upon the supposition that the rays move in straight 
lines. 

402. What is the difference between Diverging and Converging 
Rays 7 403. What is a Pencil of Rays I 404. What is a Radiant 
Point ? 405. What is the Focus of rays 1 406. What is the Inflection, 
Reflection, and Refraction of rays of light? 407. What is observed 
concerning the emission of particles of light in all directions 1 



120 OPTICS . 

Ejp. 1. Let a portion of a beam of light be intercepted by any 
boly ; the shadow of that body will be bounded by straight lines 
passing from the luminous body, and meeting the lines which ter- 
minate the opaque body. 

2. A ray of light, passing through a small orifice into a dark 
room, proceeds in a straight line. 

3. Rays will not pass through a bended tube. 

4. Rays of light may, therefore, be properly represented by 
straight lines. 

CLIX. The rays or vibrations of light move 
progressively and with -great velocity — a velocity 
much greater than that of sound ; for the flash 
of a gun, fired at a considerable distance, is seen 
some time before the report is heard. The clap 
of thunder is not heard till some time after the 
lightning has been seen. 

Obs. 1. This proposition is proved by observation made on the 
satellites of the planet Jupiter, and on the aberration of the rays 
of light from the fixed stars, from which it will be seen, that the 
velocity is at the rate of nearly 195,000 miles in one second of 
time. 

2. On account of the progressive motion of light, it is evident 
that if a luminous body were suddenly placed in the heavens, at 
the same distance, for instance, that the Sun is from us, we could 
not possibly see it before the lapse of about 8£ minutes ; and if 
the Sun were suddenly annihilated, we should not discover the 
phenomenon, until about S£ minutes had elapsed. Also, if any of 
the fixed stars were suddenly destroyed, their appearance to us 
would be the same for many years. 

Again, when we perceive a celestial object, we do not see it ex- 
actly in the place where it actually is ; but we see it in the place 
which corresponds to the change of place by the Earth, during the 
progress of light from the body to trie Earth. 

CLX. The particles of light must be exceed- 
ingly small, if they are particles ; or the force of 
the vibrations must be very delicate ; otherwise 

408. In what direction do rays of light move in the same medium ?— 
409. How is it proved that rays of light move in straight lines'? 410. 
What is the degree of velocity with which light radiates from a lumi 
nous body'? 411. How is it proved that the motion of light is progres- 
sive, and not instantaneous % 412. What curious circumstance is noticed 
as being connected with the progressive motion of light 1 



OPTICS. 121 

their velocity would render their momentum too 
great to be endured by the eye without pain. 

Exp. 1. If a candle be lighted, and there be no obstacle to 
obstruct the progress of its rays, it will fill all the space within 
two miles every way, before it has lost the least sensible part of 
its substance. 

2. Rays of light will pass without confusion through a small 
puncture in a piece of paper, from several candles in a line parallel 
to the paper, and form distinct images on a sheet of paste-board 
placed behind the paper. 

CLXT. The quantities of light, received from 
a luminous body upon a given surface, are in- 
versely as the squares of the distances of the sur- 
face from the luminous body. 

Exp. The light, passing from a candle or any luminous bed}-, 
will diverge as it proceeds, and will illuminate surfaces, which 
surfaces will be to each other as the squares of their distances 
from the candle. Thus, if at the distance of one foot, the candle 
illuminates any surface, at double the distance it will illuminate 
a surface equal to four times, and at the distance of three feet it 
will illuminate a surface equal to nine times, and, consequently," 
the surfaces illuminated are as the squares of the distances, 1,2, 
and 3. 

CLXII. If rays proceed from a radiant point 
at an infinite distance, their divergency is so tri- 
fling, that it may be considered as nothing, and 
tt*e rays may be considered as physically par- 
allel. 

Obs. I . Hence all the rays which could come from the centre, 
or any other given point, of the Sun's surface, are considered as 
parallel at the immense distance of the Earth. 

413. How is it proved that the particles of light are exceedingly mi- 
nute'? 414. What relation is observed between the quantities of light 
received from a luminous body upon a given surface, and the distances 
of the surface from a luminous body 1 415. How may this relation be 
exemplified 7 416. What is remarked concerning the divergency or 
convergency of rays of light, proceeding from a radiant point at an 
infinite - distance ? 



122 



OPTICS. 




2. To understand the nature of the convergency, divergency, 
and parallelism of rays of light, see Fig. 45. A candle at C, 
diverges rays of light towards x. They are said to converge, 
when considered as flowing from x towards C. And to be 'par- 
allel as flowing from x towards a and b. C, is the focus of the 
converging rays ; and the imaginary focus of the diverging rays. 

3. Also, if rays tend to a focus at an infinite distance, their con- 
vergency is considered as nothing, and the rays are considered as 
parallel. 

CLXIII. When rays of light pass obliquely 
out of one transparent medium into an other, 
which is either more dense, or more rare, they 
are bent out of their former course, and they are 
then said to be refracted. 

CLX1V. Rays of light are always refracted 
towards a perpendicular to the surface in a denser 
medium ; and this refraction is, more or less, in 
proportion as the rays fall more or less obliquely 
on the refracting surface. 

Exp. Let B C. Fig. 46, be supposed to be a ray of light pass- 
ing out of air into water or glass, L G, at the point C ; K K is a 
line drawn perpendicular to L G, and the ray JB C, instead of pro- 
ceeding along C H, will, in so passing, be bent towards the per- 
pendicular C, as along C I. 

CLXV. On the contrary, when light passes 
out of a denser into a rarer medium, it moves 
in a direction fa rther from the perpendicular. 

417. Explain the divergenct/j convergejicywnd parallelism of rays by 
means of a diagram. 418. When dors the refraction of light take place? 
419. When a ray <>f light passes obliquely from a rarer into a denser 
medium, which way in regard to the perpendicular is it refracted? 420. 
When a ray passes out of a denser into a rarer medium, which way- 
is it refracted 1 






REFRACTION. 



123 



Exp. I. Thus if the ray C I, Fig. 46, pass out of the glass into 
air, it wilr not proceed in C x but in the direction C B 3 farther from 
K C than x C is. 

2. Take a pan ABDC, -Fig. 47, with an upright side, into a 
dark room ; let in, by means of a small hole in a window shutter, 
a raj* of light, G B. so as to fall upon the bottom of the pan at E ; 
mark the spot E ; then, without moving the pan, fill it with water, 
and the ray now will not pass on to E, but will be refracted down 
to F. The candle G wiU answer as well as the direct rays of the 
Sun. 

3. If a shilling be stuck on the part F with wax, so that an eye 
atG can not see^it when the pan is empty, it will become visible 
the moment the pan is filled with water. 

4. Take a glass goblet half full of water, and put a shilling into 
it, then put a saucer or plate upon it, and holding it tight on, turn 
plate a^id glass together ; a by-stander unacquainted with the laws 
of refraction, will suppose that he sees a shilling and a half 
crown ; the one is seen by refraction through the water, the other 
by the rays after refraction at the surface. 

" 5. An other example of refraction maybe had by putting a stafl 
partly into the water, in an oblique position, and observicg that it 
will always appear crooked in such a position. In like manner by 
observing the bottom of vessels rilled with water, and the sandy 
bottom of rivers, appearing almost at the surface to a person who 
looks obliquely into the water. 

Obs. Rays of light which pass perpendicularly out of one me- 
dium into an other, suffer no refraction. 

CLXVI. The Angle of Incidence, is that 
which is contained between the line described by 
the incident ray, and a line perpendicular to the 
surface on which the ray strikes, raised from the 
point of incidence. Thus, in Fig. 46, the angle 
B C K is the an^le of incidence. 

CLXVII. The Angle of Reflection, is that 
which is contained between the line described by 
the reflected ray and a line drawn perpendicular 
to the reflecting surface at the point in which 
the ray passes through that surface. Thus, in 
jF/>. 46, E C K is the angle of reflection. 

421. How may those laws of refraction be illustrated by means of dia- 
grams and experiments 1 422. Why does a straight stick appear crooked 
when one end of it is immersed obliquely into the water ] 423. What is 
to be understood by the Angle of Incidence, and by the Angle of Reftec 
(ion? 



OPTICS. 



1- 



CLXVIII. Availing themselves of the princi- 
pie of refraction, philosophers have so contrived 
surfaces, that the perpendiculars to them "on 

3£l y V ^'- a l d P :° duce new and ^Portent 
effects This they have done by means of con- 
vex and concave glass lenses, so as to collect or 
Asperse the rays of light which pass through 

forms" LenSes ^ e * *»*« kinds, named according to their 

as B, Ftp. 4s. Slde ' and concave °n the other, 

A Double-convex is convex on both sides, as C Fte 4R 
A Vouble-concave is concave on both sides, as D, Fie 4 's 
E A 4T " C ° nVeX ° n ° ne Side ' and COncave on trfe other, as 
^O^^^X^^^ *«* the centre ; thus. 

CLXIX. If parallel rays fall upon a piano- 
conveniens, they will be so refracted as to unite 
in a point behind, called the principal focus, or 
jocus of parallel rays. 

k Thus the parallel rays a b, Fie- 46 falling n^ *u 
refracted towards the perpendicufar c'.JanlnStn'a 



Exam 
lens are . > 
focus at C 

Fig. 49. 




CLXX. The distance from the 
middle of the glass to the focus 
is called the Focal Distance) 
which focal distance, in a plano^ 

^^^n^X&^^^l^ Which ^ b «* * sides 



LENSES. J 25 

convex lens, is equal to the diameter of the 
sphere of which the lens is a portion, Fig. 46, 
and the focal distance of a double-convex lens 
is equal to the radius of a sphere of which the 
lens is a portion, Fig. 49. All the parallel rays 
of the Sun which pass through a convex glass 
as DE, are collected in its focus/, and the force 
of the heat at the focus is to the common heat of 
the Sun, as the area of the glass is to the area of 
the focus. 

Hlus. If a lens four inches m diameter collect the' Sun's rays 
into a focus at the distance of twelve inches, the image will not 
be more than one-tenth of an inch in diameter, the surface of this 
little circle is 1600 times less than the surface of the lens, and 
consequently the heat will be 1600 times greater at the focus than, 
at the lens. 

Cor. 1. Hence the construction of common burning-glasses, 
which are all double-convex lenses. 

2. Hence ihe reason that furniture has been set on fire by leav- 
ing a globular decanter of water incautiously exposed to the ra}'s 
of the Sun, which acts as a double-convex lens. 

Obs. The burning-glass made by Parker for Dr. Priestly, was a 
double-convex lens of flint glass, three feet in diameter, three inch- 
es thick in the middle, and weighing 212 pounds. Its focal dis-~ 
tance was six feet eight melius, and it produced a heat that melted 
iron plates in a moment. It melted 20 grains of gold in four se- 
conds — 20 grains of silver in three seconds — 10 grains of platina 
in three seconds, and as much flint in thirty seconds. 

CLXXI. If an other double-convex F G, 
Fig. 49, be placed in the rays at the same dis- 
tance from the focus, it will so refract the rays 
back again, that they will go out of it parallel to 
each other. 

423. What is meant by Focal Distance? 423. What is the rule for 
determining the focal distance of a plano-convex lens? 430. How is the 
focal distance of a double-convex lens ascertained 1 431. What is the 
proportion between the degree of heat in the. focus of a burning-glass 
and the common heat of the Sun 1 432. What kind of lens is a com- 
mon burning-glass } 433. Plow may some accidents by fire be accounted 
for'? 434. What is said of the burning-glass made by Parker for Dr. 
Priestley? 435. What effect will an other double-convex lens have, when 
placed in the refracted rays at the same distance from the focus of the 
given lens 1 

II* 






126 optics. 

Mus. It is evident that all tho rays, except the middle one, 
cross oath other in the focus/, of course the ray D A, which is 
uppermost in going in, is the lowest in going out, as G c. 

Exp. l. If a candle be placed at f, the diverging rays between 
P G, will, upon going out of the lens, become parallel at d c. 

2. 'if a candle be placed nearer the glass than the focus, the rays 
will diverge, after going through the lens. 

3. If the candle be placed farther from the glass than the focus, 
the rays will converge, after passing through the glass, and meet 
in a point which will he more or less distant from the glass, as the 
candle is nearer to, or farther from, its focus. 

4. When the rays meet, they will form an inverted image of the 
flame of the candle. Suppose B, a candle, Fig. 50, and C a con- 
vex lens, then on a dark screen, D, the image A of the candle will 
be produced, and will be reversed, because the rays cross each 
other in passing through the lens. 

Fig. 50. 





5. If an object ABC, Fig. 
51, be placed beyond the focus 
F of the glass d e f, -some of 
the rays which flow from eve- 
ry point of the object on the 
side next the glass, will fall 
upon it, and after passing 
through it they will be con- 
verged into as many points on 
the opposite side of the glass, 
where the image of the whole will be formed, which will be invert- 
ed. Thus the rays flowing from A, as A d, Ae,A/, will converge 
in the space d e'f, and bymeetiug in a will there form the image 

43G. How is this effect explained and illustrated 1 




LENSES. 127 

of the point A ; and so of those rays flowing from B and C. and of 
course of all the intermediate parts. 

6. If the object A B C, be brought nearer to the glass, the pic- 
ture a b c will be removed to a greater distance from it. 

7. The picture will be as much larger or less than the object, as 
its distance from the glass is greater or less than the distance of 
the object. 

CLXXII. When parallel rays pass through 
a double-concave lens, Jthey will diverge after 
passing through the glass, as if they had come 
from a point in the centre of the concavity of the 
glass. 

Fig. 52. 

I Exam. If the rays ab c, &c, Fig. 
71 52, pass through A B, and C be the 
~P centre of concavity, then the ray a af- 
ter passing the glass, will go on in the 
direction k I, as if it had come from c 
and no glass in the way ; the ray b will 
go on in the direction m n, and. so on. 

CLXXIII. When parallel rays pass through 
a plano-concave lens, they diverge after passing 
through it, as if they had come from a point at 
the distance of a whole diameter of the glassy 
concavity. 

CLXXIV. The following are the principal 
phenomena of rays in connexion with various 
lenses ; — 

Obs. 1. Through a convex surface, passing out of a rarer into a 
denser medium, parallel rays will become converging. 

Diverging* rays will be made to diverge less, to become paral- 
lel, or to converge, according to the degree of divergency before 
refraction, or of the convexity of the surface. 

Converging rays towards' the centre of convexity, will suffer 
no refraction. 

Converging rays tending to a point beyond the centre of con- 
vexity, will be made more converging. 

437. When parallel rays pass through a double-concave lens, what then % 
438. What is stated concerning parallel rays passing through a plano- 
concave lens'? 439. What are the principal phenomena of rays in con 
nexion with various kinds of lenses 1 






128 optics* 

Converging- rays towards a point nearer the surface than the 
centre of convexity, will be made less converging- by refraction. 

But when the rays proceed out of a denser into a rarer medium, 
the reverse occurs in each case. 

2. When rays proceed out of a rarer into a denser medium, 
through a concave surface, if parallel before refraction, they 
arc made to diverge. 

If they are divergent t they are made to diverge more, to suffer 
no refraction, or to diverge less, according as they proceed from 
some point beyond the centre, from the centre, or from some point 
between the centre and the surface. 

If they are convergent, they are either made less converging, 
parallel, or diverging, according to their degree of convergency 
before refraction ; — 

And the reverse, in passing out of a denser into a rarer me- 
dium. 

Exp. Most of the preceding propositions may be confirmed, in a 
room from which all external light is excluded, by placing a con- 
vex lens, or concave, fixed in a frame which moves perpendicularly 
upon an oblong bar of wood, or table, at different distances from a 
lighted candle placed perpendicularly on the same bar of wood, 
and receiving the images upon white paper. Upon this bar of 
wood, on one side of a line over which the convex lens is placed, 
let aline, perpendicular to the last mentioned line, be divided into 
parts, 1, 2, 3, 4, &c. each equal to the distance of the focus of par- 
allel rays ; and on the other side of the lens, let a line be divided 
in the same manner, and let the first division, which is farther 
from the lens than the focus, be subdivided into parts respectively, 
equal to £, |, &c. of the distance of the focus of parallel rays ; if 
a candle be placed over the division 2, it will form a distinct image 
on a paper held over the division § ; if a candle be over 3, the im- 
age will be at }, &c. whence it appears, that the distances of 
the correspondent foci vary reciprocally; or, by holding a large 
double-convex lens, or burning glass, in the Sun's rays, and receiv- 
ing the image on white paper, or other substance at different dis- 
tances. 

CLXXV. When rays of light strike against a 
smooth surface, and are sent back from it, they 
are said to be Reflected, and the ray that comes 
from any luminous body, and falls upon the re- 
flecting surface, is called the Incident Ray. 

440. Wfaal experiment is explanatory of those phenomena ? 441. What 
is the conseqtienu of light fall upon an opaque body, or a 

smooth surface] 442. What is the Incident Hay ; and what is the Re- 
fected Kay 7 




REFLECTION. 129 

Exam. If L G, Fig. 46, be a reflecting surface, as a looking 
glass, then B C is the incident ray, and C E is the reflected ray. 

CLXXVI. The Angle of Incidence is that 
which is contained between the incident ray, 
and a perpendicular to the reflecting surface in 
the point of reflection, as B C K, Fig-. 46. The 
Angle of Reflection is that which is contained 
between the perpendicular and the reflected ray, 
as K C E, Fig. 46. And the angles of incidence 
and reflection are always equal. 

Fig. 53. 

Obs. 1. Let the lines C a and C c, Fig. 53, 
be drawn, which are perpendicular to the con- 
cave surface, a c, and it will be found that the 
■&■** angle of incidence d a C, is equal to the angle 
"\Jj of reflection C a m, and ecCis equal to C 

C TO. 

2. Sir Isaac Newton explains the cause of reflection by suppos- 
ing, that light, in its passage from the luminous body, is disposed 
to be alternately reflected by, and transmitted through, any refract- 
ing surface it may meet with ; that these dispositions, which he 
calls Fits of easy reflection and easy transmission, return succes- 
sively at equal intervals ; and that they are communicated to it, 
at its first emission out of the luminous body it proceeds from, 
probably by some very subtile and elastic substance diffused 
through the universe, in the following manner;— As bodies fall- 
ing into water, or passing through the air, cause undulations in 
each, so the rays of light may excite vibrations in this elastic 
substance. The quickness of "these vibrations depending on the 
elasticity of the medium, as the quickness of the vibrations in 
the air, which propagate sound, depend solely on the elasticity of 
the air, and not upon the quickness of those in the sounding body, 
the motion of the particles of it may be quicker than that of the 
rays ; and, therefore, when a ray, at the instant it impinges upon 
any surface, is in that part of a vibration which is contrary to its 
motion, it may be reflected. He farther supposes, that when light 
falls upon the first surface of a bod}-, none is reflected there, but 
all that happens to it there is, that every ray that is not in a fit of 
easy transmission, is there put iuto one, so" that when they come 
at the other side, for this elastic substance easily pervading the 

443. What is the difference in quantity between the angle of incidence 
and the angle of reflection ? 444. What is Sir Isaac Newton's hypothesis 
concerning the cause of the reflection of light 7 



130 OPTICS. " 

pores of bodies is capable of the same vibrations -within the 
bo.ly as without it, the rays of one kind shall be in a fit of easy 
transmission, and those of an oilier in a fit of easy reflection, 
according to the thickness of the body, the intervals of the fits 
being different in rays of a different kind. 

3. This doctrine of fits does not accord with the general sim- 
plicity of nature, or of the other parts of this great man's philoso- 
I)hv. It seems far more reasonable to consider the phenomena of 
ight, as being produced by vibrations of a universal medium, 
the intervals between the vibrations corresponding with the fits 
above supposed. Such ideas of vibrations correspond, too, with 
the general analogies of nature in other particulars.* 

CLXXVIL The following are the principal 
phenomena of reflected rays ; — 

1 . Parallel rays reflected from a concave surface, are made 
converging. 

Converging rays falling upon a concave surface, are made to 
converge more. 

Diverging rays falling upon a concave surface, if they diverge 
from a focus of parallel rays, become parallel ; — 

If from a point nearer to the surface than that focus, they 
diverge less than before reflection ; — 

If from a point between that focus and the centre, they converge 
after reflection to some point, on the contrary side of the centre, 
and farther from the centre than the point from which they di- 
verged ; — 

If from a point beyond the centre, the reflected rays will con- 
verge to a point on the contrary side, but nearer to it than the 
point from which they diverged ; — 

445. What are tlie principal phenomena of reflected rays? 



* However obscure or ridiculous the preceding statement of Sir Isaac 
Newton's theory of fits of easy transmission and reflection, it is never- 
theless certain that at present, this subject is considered as one of the 
most curious and interesting branches of the science, having been re- 
cently improved and greatly enlarged by some of the most eminent 
French philosophers. A reviewer, in speaking of the obscure statement 
of the theory as above enunciated, has the following observation;— "The 
hypothesis of firs, however it may seem fitted to excite ridicule as exhi- 
bited in this scholium, is now justly regarded as one of the most striking 
displays which Newton ever-made of his transcendant genius. In the 
hands of Biot and his companions in the career of discovery, it has ac- 
quired an importance of which Newton himself could have had no ade- 
quate conception. — Whether the principles of this now highly interest- 
ing and important department of Optics can be reduced to the level of a 
system of elementary instruction, is deserving of serious inquiry. A 
; of the phenomena and laws of polarization, involving no difficul- 
ties which would render it inaccessible, or deprive it of Its interest with 
those who aim at nothing more than general views of science, appears at 
least to be as yet a desideratum." 



THE EYE. 131 

If from the centre, they will be reflected thither again. 

Exp. Place a concave mirror at proper distances from an 
open orifice, or a convex, or a concave lens, through which a 
beam" of solar rays passes, and verify the preceding propositions. 

2. Parallel rays reflected from a convex surface are made 
diverging. 

Diverging rays reflected from a convex surface are made more 
diverging. 

Converging ra}"s reflected from a convex surface, if they tend 
towards the focus of parallel rays, will become parallel ; — 

If to a point nearer the surface than that focus, will converge 
less than before reflection ; — 

If to a point between that focus and the centre, they will diverge 
as from a point on the contrary side of the centre, farther from it 
than the point towards which they converged ; — 

If to a point beyond the centre, they wiifdiverge as from a point 
on the contrary side of the centre nearer to it than the point to- 
wards which they converged;— 

And if towards the centre, they will proceed on reflection as 
far from the centre. 

Exp. Illustrate the three preceding propositions by receiving 
upon a convex mirror, a solar ray passing through an open orifice, 
or concave, or convex lens. 

Of the Eye. 
CLXXVIIL The eye is composed of three 
coats, covering each other, and enclosing differ- 
ent substances called humours. The three coats 
are called the Sclerotica, the Choroides, and the 
Retina. The three humours are the Aqueous, 
the Crystalline, and the Vitreous. 

Fig. 54. 

77/ us. AKG.Fig. 54, is 
L a section of the globe of the 
eye : the three circles repre- 
sent the three coats ; the ex- 
ternal firm one, which is 
white, is the sclerotica, or 
tunica-sclerotica, as it is 
sometimes called ; but the fore part, A E F B, is covered by a 
transparent coat, a little more protuberant than any other part of 

446. What experiments will illustrate or explain those phenomena? 
447. Of what is the eye composed I 4:8. What are the names of the 
three coats ; and what do they enclose ? 449. How are the coats situ- 
ated in regard to each other'? 450. What is the external covering of the 
eye called 1 




132 optics. 

the eye. and is called the tardea cornea, or simply the cornea, that 
is. transparent horny substance. The next membrane within the 
sclerotica, is the Choroides, represented by the middle circle. This 
is extended under the cornea, and forms the coloured part of the 
eye, called the /rt», which has a perforation near its middle, and 
that perforation is called the Pupil. The iris is formed of muscu- 
lar fibres, disposed in two direct ions, that is, some are like radii, 
tending towards the centre, and others are circular. The iris is 
likewise tinged with a variety of colours, from which the eye is 
said to be blue, hazel, black, &c. 

At the hinder part of the eye, but not exactly opposite to the pu- 
pil, a prolongation of the coats of the eye is to be observed, as at d. 
This prolongation envelopes a nerve that comes from the brain, 
and is called the Optic Nerve, the inner or medullary part of which 
-spreads itself over the choroides, and forms the innermost coat of 
the eye, called the Retina, which is a thin whitish membrane, 
having the appearance of the finest sort of net- work, or of linen, 
and serves to receive the images of objects produced by the refrac- 
tion of the different humours of the eye. 

The crystalline humour, y z, is a consistent cellular transparent 
substance, in the shape of a double-convex lens, the anterior sur- 
face of which is less convex than the other surface. This humour 
is kept, in its place, and is fixed to the coats, by being attached all 
round by certain ligaments, called Ligamenta Ciliaria. All the 
cavity, AEFB x. between the cornea and the crystalline hu- 
mour, is filled with a thin transparent fluid, like water, and hence 
called the Aqueous Humour. The Vitreous Humour, which is a 
transparent substance, neither so fluid as the aqueous humour nor 
so hard as the crystalline, occupies all the interior part of the eye 
??i 77 behind the crystalline. 

Obs. Mr. Harris has given a table of the dimensions of the parts 
of the human eye, of which the following are the principal parti- 
culars in parts of inches. 

Diameter from the cornea to the choroides . . .95 

Radius of the cornea .335 

Distance of the cornea from the first surface of the 

crystalline IOC 

Radius of the first surface of the crystalline . . .331 

Radius of the back surface of the crystalline . . .25 
Thickness of the crystalline . . . . . .373 

CLXXIX. Objects are seen in consequence 
of the images' being painted on the nerves, or 

451. Which part of the eye is called the Cornea? 452. What is that 
part of the eye called through which the light passes'} 453. Bywhatis 
the pupil of the eye surrounded 7 451 In what part of the eye is the sense 
of vision situated ? 165. What is the name of the prolongation at the back 
part of the eve? 450. What are the three humours called ; and how are 
thry situated? 



THE EYE. 133 

retina, at the back of the eye ; and though the 
images of objects are painted on the retina 
inverted, yet they are seen erect by the habit of 
the mind. 

rdus. As the object ABC, Fig. 54, sends out rays that fall on 
the cornea of the eye between E and F, and by passing- on through 
the pupil and humours, they will be converged to as many points 
on the retina, and will there form a distinct inverted picture, cb a 
of the object. Thus the pencil of rays, qr s, that flows from A, 
will be converged to the point a on the retina ; those from B, will 
be converged to the point b; those from C to the point c; and so 
of all the Intermediate points; by which means the whole picture 
a b c is formed, and the object is made visible. 

Eiy. Take off the sclerotica from the back part of the eye of an 
ox, or other animal, and place the eye in the hole of the window 
shutter in a dark room, with its fore part towards the external ob- 
jects ; a person in the room will, through the transparent coat, see 
the inverted image painted upon the retina. 

Obs. 1. It is found from experience, that when the image upon 
the retina is bright, the object is clearly seen ; and when the image 
is faint, the object appears faint ; also, that when the image is dis- 
tinct, the object is seen distinctly ; and when the image is con- 
fused, the object appears confused. Hence it may be concluded, 
that these images are the cause of vision. 

2. It is manifest, that a different conformation of the eye, or 
some parts of the eye, is necessary for distinct vision at different 
distances. Some think the change is in the length of the eye ; 
others, that it is a change in the figure or position of the crystal- 
line humour; and others, that it is a change in the cornea; but 
any of these changes would produce the effect. 

CLXXX. Dimness of sight generally attends 
old people, and arises either by the eye's growing 
flat, and not uniting the rays exactly on the re- 
tina ; or by the humours' losing their transpa- 
rency in some degree, which makes every object 
appear faint and indistinct. 

CLXXXI. If the crystalline humour has either 
too much or too little convexity, the sight will 

457. How is it proved that the images of objects are represented or 
painted on the retina in an inverted position? 458. In what way can it 
be proved that the images upon the retina are the cause of vision? 459. 
What is the cause of dimness of sight in old age 1 

12 



134 optics. — 

be defective, owing to the image being formed 
before or behind the retina. 

Obs. In myopes, or persons who are short-sighted, the hu- 
mours of the eye are too convex, and bring the rays to a focus, 
before they reach the retina, unless the object be brought near to 
it ; in which case, the image is cast farther back. In others, the 
humours of the eye have so little convexity, that the focal point 
lies behind the retina; whence, unless the object is removed to a 
greater distance from the eye, the vision will be indistinct. Such 
persons are called Presbytce. 

CLXXXII. When the diameter of an object 
is given, its apparent diameter to the eye, is in- 
versely as its distance from the eye, and the ap- 
parent diameter is the angle which it subtends 
to the eye ; so that its apparent size is as the an- 
gle which it subtends to the eye. 

Obs. l. The angle subtended as the least visible object, called 
by the writers on Optics the Minimum Visible, can not be accu- 
rately ascertained, as it depends upon the colour of the object, and 
the ground upon which it is seen ; it depends also upon the eye. 
Mr /Harris thinks the least angle for any object to be about 40" ; 
and at a medium, not less than 2'. To the generality of eyes, the 
nearest distance of distinct vision, is about 7 or 8 inches. Taking 
8 inches for that distance, and 2' for the least visible angle, a glo- 
bular object of less than the three-hundredth part of an inch can 
not be seen. 

2. The apparent diameter of an object is as^the diameter of its 
image upon the retina ; and the diameter of the image, when the 
object is given, is inversely as the distance of the object ; therefore 
the apparent diameter of the object is also inversely as the dis- 
tance of the object. The same may be proved of any apparent 
length whatsoever. 

3" Hence the apparent diameter of an object maybe magnified 
in any proportion ; for the less the distance of the eye from the 
object, the greater will be its apparent, diameter. But without the 
help of glasses, an object brought nearer the eye than about five 
inches, though it appears larger, will at the same time appear 
confusedly. 

4. Many deceptions in vision arise from the above consideration. 
We judge of the distance of any object by the visible length of the 

4G0. What peculiarity or (Infect of the eye causes some persons to be 
short-sighted i 461. What is the proportion between the apparent diame- 
t r of an object and its distance from the eye % 



COLOURS. 135 

plane, which lies between the eye and the object. When this me- 
thod fails us, we compare the known magnitude of the object with 
its present apparent magnitude ; or we conjecture the degrees of 
distinctness with which we see the several parts of an object ; or 
we observe whether the change of the apparent place of an object 
when viewed from different stations, or its parallax be great, or 
small, this change being always in proportion to the distance of 
the object. On this principle, we may judge of the distance of a 
near object, by observing the change which is made in its appa- 
rent situation, upon yie Wing it successively w T ith each eye singly. 
Or, since it is the difference of the apparent place of an object, as 
viewed by each eye separately, which causes an object to be seen 
double, unless we turn both eyes directly towards it, and since, in 
doing this, where the distance is very small, we turn the eyes very 
much towards each other, and less at a greater distance ; the dif- 
ferent sensations accompanying the different degrees in which the 
eyes are turned towards each other, afford by habit, a rule for 
judging of the distance. 

5. In objects placed at such distances as we are used to, and 
can readily allow for, we know, by experience, how much an in- 
crease of distance will diminish their apparent magnitude, and, 
therefore, instantly conceive their real magnitude, and neglecting 
the apparent, suppose them of the size they would appear^if they 
were less remote ; but this can only be done, where we are well 
acquainted with the real magnitude of the object ; in all other 
cases, we judge of magnitudes by the angle which the object sub- 
tends at the known, or supposed, distance ; that is, we infer the 
real magnitude from the apparent magnitude in comparison with 
the distance of the object. 

OF COLOURS. 

CLXXXII1. Rays of light are differently 
refrangible, or refractible, that is to say, some 
are more easily turned out of their course than 
others ; and are differently reflexible, when some 
are more easily reflected than others. 

Obs. 1. The powers of rays in regard to their refrangibility or 
reflexibility, are ascribed, on the hypothesis of their being parti- 
cles like sand, to the different momenta of the particles ; and on 
the hypothesis of their being simple vibrations like undulations of 
w T ater, to the vivacity or acuteness of the vibrations. It seems, 
indeed, evident, that the rays which are turned the most out d 

462. How do we judge of the distance of an object? 463. Are all the 
/ rays of light equally refrangible and rejlexible 7 464. Why are not all 
the rays of light refractible and reflexible alike 1 



136 optics. 

their course by a refrangible medium have less powers or momenta, 
than those which are least diverted from their course. Different 
colours arc 1 therefore simply the effect of the different perceptions 
produced on the optie nerves by the different forces or the action 
of the rays, the red being the most forcible, and the violet the least. 
2. The rays which are reflected, from opaque bodies, always 
bring with them to the eye the idea of colour; and this principle 
of colour, in different bodies, is simply a disposition to reflect to 
the eye more copiously one kind of rays than an other. For par- 
ticular rays are originally endowed with particular colours ; some 
are red, others blue, others yellow, and others green, &c. 

CLXXXIV. Light is called Homogeneous, 
when all the rays are equally refrangible ; and He- 
tcrogeneous.when some rays are more refrangible 
than others. The colours produced by homoge- 
neou s rays, are called Primary or Simple colours ; 
those of heterogeneous, Secondary or Mixed. 

Obs. Every ray of light, as it emanates from the Sun or from 
any luminous body, seems to be a combination of all the simple 
elementary rays ; and as some of them are more refrangible than 
others ; that is, are more susceptible of being turned out of their 
course, in passing from one medium into an other ; it is evident, 
that after such refraction, those rays will be separated, and their 
colours observed. Of these, the most refrangible are the violet, 
and the least refrangible the red rays; and the intermediate ones, 
in order, are indigo, blue, green, yellow, and orange. 

CLXXXV. To examine the different colours 
of a ray of light, a small hole may be made in 
the shutter of a dark room, and the ray must 
fall upon a prism in an oblique direction. 

Obs. Prisms are commonly made of solid glass, but students 
who do not possess one of this kind, may easily make a substi- 
tute. Take three pieces of plate glass, the kind of which looking 
glasses are made, each four or six inches long, and two or three 
inches wide ; get made of tin a frame consisting of the two ends 

465. By what kind of rays do we see opaque bodies ; and what is the 
effect of the reflection of li<rht from opaque bodies ? 466. What is to be 
understood by Homogeneous light ; and what is meant by Heterogene- 
ous Light ? 467. What are the colours produced by homogeneous rays 
called ? 463. What are Secondary, or Mixed colours 1 46 I. What ia 
fiaid of the heterogeneity and refrangibility of every ray of light 1 470. 
How is a ray of light decomposed, or separated into its primary colours 7 



PRISMATIC REFRACTION. 137 

in the exact shape of the three pieces of glass placed in the form 
of a triangle, with a strip of tin running from each angle of one 
end to the angles or corners of the other. These strips are bent 
so as to receive the two edges of the glass plates. When the 
frame is complete, except the soldering in one of the ends, fix the 
glass plates in their places, and then have the end soldered. The 
tin forming the ends is turned up so as to receive the plates, and 
one of the ends is furnished with a spout to pour in water. When 
this is done, the vessel is made water-tight, by stopping with putty- 
all the crevices between the glass and tin. The prism is then filled 
with clear water by the spout, and corked up. 

Fig. 55. Fig. 56. 




Ulus. 1. Let A B, Figure 55, represent part of the shutter of 
the window of a room, in which no light enters except through 
the hole C. If the light of the Sun be received upon a screen at 
any distance from the hole, as at E, a circular luminous spot will 
be formed upon the screen, which is larger in diameter than the 
hole at C. Place a glass prism E before the hole, so that the 
light may pass through it in a direction perpendicular to the axis 
of the prism ; and, instead of going straight from E to F,the light 
which comes through the hole, will, by passing through the prism, 
be bent and dispersed in such a manner, as to form a coloured spec- 
trum or image G H upon a screen which may be situated at any 
distance from the prism, but below the straight direction C F. — 
The length of the spectrum G H is equal to about five times its 
hreadth, and is terminated by semicircular ends. The highest 
part G is of a beautiful red. colour, which, by insensible shades, 
degenerates into an orange^ then a yellow , a green, a blue, an 
indigo, and a violet, whicn is the colour next to H, or at the lowest 
part of the spectrum. 

2. Or, in Fig. 56, let E G be a shutter, F a hole, S S a ray or 
direction of solar light, A B C a glass prism ; then the light fail- 
ing on B C will be unequally refracted to the wall M N, and pro- 
duce the coloured spectrum P T. This oblong image is of dirTer- 

471. How does a prism separate or disperse, the rays of light ? 472. 
What are the colours of the prismatic spectrum; and in what order do they 
succeed each other 1 473. Which rays deviate most and which least 
from their original course, in passing through a prism 1 
12* 



138 optics. 

cnt colours in different parts, the whole image being made up of 
rays of seven different colours, in the following order, beginning 
with those which arc most refrangible ; violet, indigo, blue, green, 
yclloii\ orange, red. This refracted picture consists of several 
round pictures so near to each other, that each higher circle mixes 
in part with that below it, whence the colours near the upper and 
lower edge of each circle are blended, the sides of these circles, 
being very near to each other, appear like straight lines. 

CLXXXVI. If the whole spectrum be divided 
into 360 parts, the red will occupy 45 of them, 
the orange 27, the yellow 48, the green and the 
blue 60 each, the indigo 40, and the violet 80. 
By mixing the seveix primitive colours in these 
proportions, a dusky white is obtained. — And 
the seven colours are reducible to three, the red, 
blue, and yellow. 

Exp. Paint on a circular board the seven colours in their proper 
proportions, extending in sectoral spaces from the centre ; and then 
whirl the board with great velocity, it will appear of a dirty 
white. If the colours are more neatly laid on and accurately de- 
fined, the white will appear more resplendent also. 

CLXXXVII. The colours of homogeneous 
light can neither be changed by refraction nor 
reflection ; — and the whiteness of the Sun's light 
arises from a composition of all the original or 
primary colours mixed in a due proportion. 

Exp. 1. Let a beam of homogeneous light pass through around 
hole in a pasteboard, and then.be refracted by a prism on the other 
side, the colour of the rays will remain the same. 

2. Red lead, viewed in homogeneous red light, will be red, but 
if placed in green, or any other homogeneous light, it will take 
the colour of the rays which fall upon it. 

CLXXXVIII. The colours of all bodies are 
either the simple colours of a homogeneous light, 

471. Whnt fact, s mentioned respecting a painted board or circular 
card, as proving that the seven primary colours constitute white when 
combined together 1 475. Of what is the whiteness of the Sun's light. 
composed 1 476. Whatcflect does refraction or reflection have in chang- 
ing the colours of homogeneous light 1 



PRISMATIC REFRACTION. 139 

or such compound colours as arise from the mix- 
ture of homogeneous light. 

Obs. 1. Each sort of light having a peculiar colour of its own, 
which no refraction or reflection can alter, since bodies appear 
coloured only by reflected light, their colours can be no other than 
the colour of some single homogeneous light, or of a mixture of 
different sorts of light. 

2. When the thickness of the particles of a body is such, that 
one sort of colour is reflected, other colours will be transmitted, 
and therefore the body will appear of the first colour. And, in 
general, a less thickness is found to be necessary to reflect the 
most refrangible rays, as violet and indigo, than "those which are 
least refrangible, as" red and orange. Sir I. Newton, from a variety 
of experiments on light and colours, concludes that every substance 
in nature is transparent, provided it may be sufficiently thin. 

3. Mr. Delval has, however, by a great variety of well conduct- 
ed experiments, shown that colours are exhibited, not by reflected, 
but by transmitted light. This he proved by covering coloured 
Sflass, and other transparent coloured media, on the further sur- 
face, with some substance perfectly opaque, by which he found 
that they reflected no colour, but appeared perfectly black. He 
concludes, therefore, that, as the fibres of mineral and animal 
substances are found, when cleared of heterogeneous matters, to 
be perfectly white, the rays of light are reflected from these white 
particles, through the coloured media with which they are cover- 
ed ; that these media serve to intercept and impede certain rays in 
their passage through them, while a free passage being left to 
others, they exhibit, according to these circumstances, different 
colours. 

4. Mr. Delval concludes, (l) That the colouring particles do not 
reflect any light. (2) That a medium, such as Sir I. Newton de- 
scribed, is diffused over the anterior and further surfaces of the 
plates, whereby objects are reflected equally and regularly as in a 
mirror. 

5. To determine the principle on which opaque bodies appear 
coloured, it must be recollected, first, ihat all the coloured liquors 
appear such only by transmitted light ; and, secondly, that these 
liquors, spread thinly upon a white ground, exhibit their respect- 
ive colours ; he therefore concludes that all coloured bodies, 
which are not transparent, consist of a substratum of some white 
substance, which is thinly covered with the colouring particles. 

6. On extracting, by means of spirits of wine, the colouring 
matter from the leaves, wood, and other parts of vegetables, he 
found that the basis was a substance perfectly white. He also 

477. In what manner are the colours of all bodies constituted, or ac- 
counted for 1 473. How may the whiteness or blackness of bodies be ex- 
plained 1 



140 OPTICS. 

extracted the colouring matter from different animal substances, as 
flesh, feathers, &c. when the same conclusion was obtained. Flesh 
consists of fibrous vessels, containing blood, and is perfectly white 
when divested of blood by ablution, and the red colour proceeds 
from the light which is reflected from the white fibrous substance 
through the red transparent covering formed by the blood. The 
result was the same from an examination of the mineral kingdom. 

OF THE RAINBOW. 

CLXXXIX. The Rainbow is a meteor in the 
form of a party-coloured arch, or semicircle, exhib- 
ited only during the time of rain. It is seen in that 
part of the heavens which is opposite to the Sun, 
and is occasioned by the refraction and reflection 
of the solar rays in the drops of falling rain. There 
is likewise, though not always distinctly visible, 
a secondary, or less brilliant rainbow, investing 
the former at some distance. 

Obs. l. The colours of the inner or primary bow are much more 
vivid than those of the secondary bow ; and each bow exhibits all 
the prismatic colours, arranged in the same manner as in the pris- 
matic spectrum — violet, indigo, blue, green, yellow, orange, and 
red ; but the order of those colours in the exterior bow is contrary 
to that of the inner bow, the latter having the violet below and the 
red above, whilst the former has the red below and the violet 
above. Those colours are blended into each other, in such a man- 
ner that the eye can not distinguish their boundaries ; and indeed, 
for most e)*es, it is difficult to distinguish more than three or four 
principal colours. 

The situation of the rainbows changes according as the eye of 
the spectator changes situation ; for otherwise, their breadths, &c. 
could not subtend constantly the same angles, which they uni- 
formly do ; hence no two persons can ever see the same rainbow 
precisely, or the same colour in the very same portion of the 
heavens. — 

2. When the Sun shines upon drops of rain as they are falling, 
the rays which come from those drops to the eye of the spectator, 

479. Of what colour is the colouring matter of various substances, as 
the leaves of trees, 6cc. ] 48). What are Mr. Delval's conclusions 1 481. 
What is the Rainbow ; and how is it produced 1 482. What is said ol 
Jae splendour and arrangement of the colours in the primary and in the 
secondary bow 1 483. Can any two persons ever see the same rainbow 
precisely! 481. Why not 7 



THE RAINBOW. 141 

after one reflection and two refractions, produce the interior or 
primary rainbow. 

A ray of light coming from the Sun and falling upon a drop of 
rain, is refracfed towards the centre on the farther side, from which 
it is also reflected and again refracted on passing out of the drop. — 
And as light is differently refrangible, the last refracted rays will 
make different angles with the incident rays ; the angle which the 
effectual red rays make with the incident rays, being found to h# 
42° 2Q1 and that of the violet rays 40° 17' . 

Exp. Let a glass globe, filled with water, be exposed to the rays 
of the Sun ; let the eye of the spectator be so situated, that the last 
refracted ray from the drop, coming to the eye, shall make an angle 
of about 42° with the line passing through, the eye .and the Sun, 
the red rays only will be seen. If the place of the eye be changed, 
so as to enlarge this angle, the red rays will disappear ; but if the 
angle be diminished, the colours of the more refrangible rays will 
appear. 

Obs. 1. Cascades and fountains, whose waters, in their fall, 
are divided into drops, will exhibit rainbows to a spectator, pro- 
perly situated, during the time of the Sun's shining. This appear- 
ance is also seen by moon -light, though seldom sufficiently vivid 
to render the colours distinguishable. Coloured bows have been 
seen on grass formed by the refraction of the Sun's rays in the 
morning dew. 

Such a coloured bow is not unfrequently seen at sea in the spray 
or drops of water which the wind disperses or carries away from 
the tops of the waves ; but the colours are not so vivid as those of 
the common rainbow; the most vivid being a yellow next to the 
Sun, and a green next to the sea. 

2. Artificial rainbows may be produced by a candle-light on the 
drops of water ejected by a small fountain, or jet d'eau, or from 
the stream emitted from an eolipile. But the most natural and 
pleasing is by means of the air-fountain, the jet of which is perfo- 
rated with a great number of very fine holes from which the water 
spouts so as to form a kind of fluted column. The rainbow is 
formed by the Sun's rays, for the spectator has to place the spout- 
ing streams directly in the Sun's beams, with his own back to the 
Sun, and being in a direct line with the Sun and the centre of 
the jet. by stooping his head to a certain degree, he will discover 
the beautiful appearance of the natural prismatic colours, and a 
small rainbow, on the same principle as those which are seen in 
t the time of rain and sun-shine. 

CXC. The primary rainbow can never be 
greater than a semicircle. 

485. In what manner are the rays of light refracted and reflected, so 
as to produce the primary rainbow? 486. In w T hat manner may arti- 
i ficial rainbows be produced ? 457. Why is a primary rainbow never 
greater than a semicircle? 




142 optics. 

Obs. For the line drawn from the Sun through the eye o 
spectator, passes also through the centre of the rainbow, which 
centre is always opposite to the Sun. And since this line makes 
an angle of 42° 20' with the reflected or effectual red ray, the high- 
est point of the bow would be more than 42° 20' from its centre. 
If, therefore, the Sun is more than 42° 20' above the horizon, the 
centre of the bow, which is opposite to the Sun, must be more than 
42° 20' below the horizon, and no primary bow will be seen. 

CXCI. When the light falls upon the under 
parts of the drops of rain, some of the rays, after 
two reflections and two refractions, come to the 
eye of the spectator who had his back towards 
the Sun, and his face towards the drop. And 
those rays which are parallel to each other, after 
they have been once refracted and once reflected 
in a drop of rain, will be coloured, and produce an 
exterior rainbow, as soon as they emerge after 
two refractions and two reflections. 

Obs. The angle which the violet rays make with the incident 
ones, is found to be 54° T and that of the red rays 53° 57'. 

CXCII. The colours of the secondary rainbow 
are much fainter than those of the primary bow, 
and are ranged in the contrary order. In the 
latter, the red rays are outermost and the violet 
innermost ; but in the former, the red rays are 
innermost and the violet outermost. 

Obs. At every reflection, many rays pass out of the drop with- 
out being reflected : consequently, the secondary rainbow, which 
is produced after two reflections, is formed by fewer rays than the 
first, which is produced after one reflection. 

OF OPTICAL INSTRUMENTS. 

CXCTII. A Mirror or Speculum, is an opaque 
body, the surface of which is finely polished, so 
that it will reflect the rays of light which fall 
upon it, and thus represent the images of objects. 

488. How is the appearance or production of the secondary rainbow 
explained and illustrated \ 489 What is a Mirror or Speculum, 1 



OF MICROSCOPES. 143 

Obs. Mirrors are made of metal, or glass polished on one side 
and silvered on the other. There are three kinds of mirrors, — 
the plane, the convex, and the concave. 

Exp. If the parallel rays, Fig. 53, d a, C m b, and e c, fall upon 
the concave mirror A B, then da will be reflected along am,Cb 
will be reflected along bm, and e c along c m; of course they all 
meet in m ; and m b is found to be equal to m C, or half C6. 

CXCIV. Concave glasses are necessary to 
those whose eyes are too convex. 

lllus. When the eye is too convex, the rays proceeding from 
objects are converged to a focus before they get to the retina; to 
remedy this, a concave glass is used, because rhe property of this 
is, to disperse the rays which prevent them from coming to a focus 
so soon as they otherwise would. 

CXCV. Convex glasses are necessary to those 
whose eyes are too flat. 

Ulus. When the eye is too flat, the rays proceeding from objects 
do not converge to a focus as soon as {hey reach the retina ; a 
convex glass has the property of converging the rays, and, of 
course, bringing them to a focus sooner than they otherwise 
would. 

Obs. Were there no other use of the science of Dioptrics, says 
Mr. Molyneux, than that relating to spectacles, the advantage - 
that mankind receive thereby is inferior to no other benefit, not 
absolutely requisite to the support of life. For as the sight is the 
most noble and extensive of all our senses ; as we make the most 
frequent use of our eyes in all the actions and concerns of life ; 
surely that instrument which relieves the eyes when decayed, and 
supplies their defects, must be estimated as the greatest of all 
advantages. Forlorn, indeed, must have been the situation of 
many young, and almost all old people, before this admirable 
invention. The same author concludes that spectacles were first 
used about the end of the 1 3th century, and he ascribes the inven- 
tion to Friar Bacon. Others suppose spectacles to have been in- 
vented by Spina, of Pisa, in the year 1299. 

CXCVI. Microscopes are instruments for 
viewing small objects ; and they apparently 
magnify objects, because they enable us to see 
them nearer than with the naked eye, without 
affecting the distinctness of vision. 

490. On what principle do concave glasses assist the eye-sight T 491. 
How do convex glasses assist the sight 1 492. What is said of the utility 
of Dioptrics 1 493. In what way can objects be seen distinctly when 
placed near the eye? 494. What is a Microscope! 






144 optics. 

Exp. 1. Take a piece of brown paper and make a pin-hole in it, 
then bring the eye close to the hole, and the paper within two or 
three inches of any small print or object, and it will be apparently 
much magnified, though without the paper the letters would at 
that distance be wholly illegible. 

Obs. All that the hole or microscope effects, is, to enable us to 
see an object distinctly much nearer to the eye than it could be 
seen by the eye unaided. The magnifying power is as the pro- 
portion of the distance, at which we usually view objects, to that 
at which the microscope enables us to see them or their true im- 
ages. 

CXCVII. Microscopes are usually reckoned 
to be of three kinds, the Simple, the Compound, 
and the Solar. 

i. The Simple microscope, is a small double- 
convex lens, having the object placed in the 
focus, and the eye at the same distance on the 
other side. Its magnifying power is found by 
dividing seven inches, the least distance at which 
an object can be seen distinctly by the naked 
eye, by the focal distance of the lens. 

Exp. 1. If the focal distance of the lens be only the \ of an inch, 
then the diameter of an object will be magnified 28 times, because 
7 divided by \ is the same as multiplying 7 by 4, and the surface 
will be magnified 784 times. 

2. The most powerful simple microscopes are very small glo- 
bules of glass, which any curious person may make for himself, 
by melting the ends of fine threads of glass in the flame of a can- 
dle, or by taking a little finely pulverized glass upon the point of 
a very small needle, and melting it into a globule in the same 
way. It was with such microscopes that the renowned microsco- 
pic observer, Leewenhoeck made all his wonderful discoveries, 
most of which are deposited in the British Museum. This noted 
character, it is said, employed almost the whole of his life in mi- 
croscopical observations. 

3. Provide a plate of lead, about the thirtieth of an inch in thick- 
ness, and make a round perforation in it by means of a large 
pin or needle, pare the edges of the hole, and put into it, with the 
point of a feather, a small drop of water. The two surfaces of the 

49o. Why do small objects appear larger through a microscope than with 
the naked eye ? 190. Of what docs a Single or Simple microscope consist 1 
497. What is the law or rule in regard to estimating the magnifying 
power of a microscope ? 498. How are the most powerful simple micro- 
scopes made 1 499. What is said of Leewenhoeck, his microscopes, and 
his microscopical observations ? 



OF MICROSCOPES. 145 

water will assume a convex spherical form ; and by applying it 
close to the eye, and the object to be viewed, the latter will be very 
much magnified. A contrivance of this kind is usually known as 
a Water Microscope. 

ii. The Compound microscope consists of an 
object-glass and an eye-glass. Its power is in 
proportion as the image is larger than the object, 
and also according as we are able to view it at a 
less distance. There are generally two eye- 
glasses, by which means the object is'less magni- 
fied, but more of it is seen. 

° ' ' lllus. 1. The object to be viewed is 

o 1 p» p* "IS /wS^*\ a ^' Fig. 59, c d is the object-glass, 

V« Jf?$^' and e ./the eye-glass. The small 

t^ ^^^ Mjrll^^i^^^lJ object a. b, is placed a little beyond 
£ J ^^^N^^^l^^/ the focus ofrfc, the rays will con- 
^^^^S verge and the image be formed a.igh. 
The image, therefore, and not the object, is viewed by the eye, B 
A E, through the lens ef> which is so placed that the image gh 
may be in its focus, and the eye about the same distance on the 
other side ; the rays on each pencil will be parallel after going out 
of the eye-glass at ef, till they come to the eye at k, where they 
will begin to converge by the refractive humour of the eye, and 
having crossed each other and passed through the crystalline and 
vitreous humours, they will form the inverted image' A B on the 
retina. 

2. If the image g h is 4 times larger than the object a b, and 
by the help of the eye-glass we can view it 7 times nearer than 
we could by the naked eye, on both these accounts the diameter of 
the object will be magnified 4 times seven, cr 23 times j and the 
surface 23 times 23, or" 734 times. 

in. The Solar microscope depends on the 
sunshine, and is used in a darkened room. It 
is composed of a tube, a looking-glass, a convex 
lens, and a single microscope. The Sun's rays 
are reflected by the looking-glass through the 
tube upon the object, the image of which is 

50). What is tlie construction of a Water Microscope! 501. Explain 
the use and construction of the Compound or Double microscope. — 
50*2. How is its magnifying power computed? 503. Upon what doe.* 
the Solar microscope depend ; and how is it used? 



146 optics. 

thrown upon a white screen, sheet, &c. placed 
at a distance to receive it, 

E.rp. In the window-shutter of a dark room, let a round hole 
be made about three inches in diameter, through which the Sim 
may east a cylinder of rays into the room, and place in it a glass 
lens of about 12 inches focal distance and 2 inches in diameter. — 
To the inside of the hole adapt a tube, having previously connect- 
ed it with the glass lens. In the tube, at a small distance from 
the lens, a slit maybe made, capable of receiving one or two very 
thin plates of glass, to which the object to be viewed may be affixed 
by means of a little gum water, exceedingly transparent. Into 
this tube fix an other, furnished at its anterior extremity with a 
convex lens, of half an inch focal distance ; and placet! at tlie 
same distance from the object. Let a plane mirror, connected 
with the tube, and moveable by means of a wheel, receive the 
Sun's rays on the outside of the shutter, and convey them into 
the tube. The rays passing through the first lens, will strongly 
illuminate the object, from which they will pass through the sec- 
ond lens, and form an inverted image of the object, very distinctly 
on a card or a piece of white paper, placed at a proper distance. — 
This image will appear to be greatly magnified, that is, in the ra- 
tio of the distance of the object from the lens to that of the image 
from the lens. 

A small insect will appear a large animal, or a hair as big as a 
walking-stick ; The eels in vinegar, or in flour paste, will appear 
like small serpents. Animalcula may be thus discovered in vari- 
ous infusions, as of leaves, Brazil wood, and other vegetable mat- 
ters. These experiments may be directed to innumerable objects ; 
as, the mites in cheese ; the movements of the Tremella or gela- 
tinous green plant which forms itself in stagnant water; the cir- 
culation and globules of the blood ; the pores and scales of 
hair, the pores of the hands, after being well abluted w~ith soap 
and water ; the eyes of most insects, as of a fly ; the webs of 
spiders ; the louse and flea ; mouldiness ; the dust of the lycop- 
erdon, or pufT-ball; farina of flowers ; apparent holes in the leaves 
of some plants, as the hypericum, or St. John's wort ; down of 
plants ; the globules produced from the collision of flint and steer; 
the asperities of various polished surfaces; the edge of a razor, 
or point of a needle; pores of charcoal, &c. 

Obs. The magnifying power of this instrument is in proportion 
as the distance of the image from the object-glass is greater than 
the distance of the object itself from it. If the distance of the ob- 
ject from the object-glass be £ of an inch, and the distance of the 

501. In what manner may a solar micrnsropo be fornW. 505. What 
are its effects : ami what substances are the most proper objects for mi- 
croscopical experiments 1 606. What is the rule for ascertaining the 
mogniiying power of a solar microscope? 



TELESCOPES. 147 

picture be ten feet or 120 inches, then the object is magnified in 
length 4S0 times, or in surface 230,000. 

CXCVIII. Telescopes are used for viewing 
objects at a great distance ; of these there are two 
kinds, the Refracting and Reflecting. 

CXC1X. It is the sole business of all Tele- 
scopes to enable the eye to see distant objects 
under an enlarged angle. For this purpose, a 
new image of an object is produced by the object- 
glass of the telescope, and then this image is 
viewed by means of the eye-glasses. 

Obs. As the object or its image is seen by the eye under an en- 
larged angle, or in the same way in which it would be seen if much 
nearer to the eye, so the first impression conveyed to the mind by 
a telescope, is that of bringing- the object nearer, which is only an 
other mode of declaring that it is enlarged, or seen under a large 
angle. 

CC. The apparent diameter of an object seen 
through an astronomical telescope,* is to the ap- 
parent diameter of the same object seen by the 
naked eye, as the distance of the image from 
the object-glass is to its distance from the eye- 
glass. 

Illus. If the image, formed b} r the object-glass were received 
upon paper, the apparent diameter of the object seen by the 
naked eye at the station of the object-glass, would be equal to 
the apparent diameter of the image seen from the same station, and 
the apparent diameter of the image w^ill of necessity be inversely 
as the distance of the eye from it, or as the focal distance of 
the object-glass. If the eye then be placed at the station of 
the eye-glass, consequently, the image will appear to the eye in 

507. Point out the use of telescopes ; how many kinds ; and the differ- 
ence between the Refracting and" the Reflecting telescope. 50^. What 
is the sole business, effect, or design of all telescopes 1 509. What is 
the reason that the image of an object is seen through a telescope 
under an enlarged angle 7 510. \Wiat is an Astronomical telescope 7 

* An Astronomical Telescope consists of two convex lenses, whose 
distance from each other is equal to the sum of their. principal focal 
distances ; that lens which is towards the object, is called the Object- 
glass ; that whieh is next to the eye is called the JEye-glass. 






H8 optics. 

that nearer station bigger than at the object-glass, in the inverse 
ratio of the distances. Therefore the apparent diameter of the 
object seen with the telcseopc, is to the diameter of the same 
object seen by the naked eye at the station of the object-glass, 
as the distance of the distinct image from the object-glass is to 
the distance from the eye-glass, that is, as the focal distance of 
the object-glass is to the focal distance of the eye-glass ; conse- 
quently, if the former be divided by the latter, the quotient will 
express the magnifying power ; thus, if the telescope will magnify 
ten times in diameter, the focal distance of the object-glass is ten 
inches, and that of the eye-glass one inch. 

Obs. 1. Consequently a telescope will not magnify an object, 
unless the focal distance of the object-glass is greater than the focal 
distance of the eye-glass. And, of course, the object-glass of a 
telescope should be Jess convex than the eye-glass. 

2. An object will be equally magnified by two telescopes of very 
different lengths, if the ratio of the focal distances of the object- 
glass and the eye-glass be the same in each. 

3. And if a telescope is inverted, objects seen through it will be 
diminished ; for the object-glass which has the greater focal dis- 
tance then becomes the eye-glass, and it reverses the proportion. 

4. The visible area, or space which may be seen at one view 
through a telescope, is as the area of the eye-glass. 

5. The brightness of an object seen through a telescope, depends 
upon the area of the object-glass, but not the visible area. 

6. The distance of the eye from the eye-glass 3 should be equal 
to the principal focal distance of the eye-glass. 

CCI. A telescope, which represents objects in 
their natural posture, should have three eye- 
glasses.* The two additional lenses serve sim- 
ply to give an erect position to the images of ob- 
jects. The three eye-glasses have all their focal 
distances equal, and the magnifying power is 
found as before, by dividing the focal distance of 

511. What kind of proportionality subsists between the apparent 
diameter of an object seen through a telescope, the apparent diameter of 
the same object seen by the naked eye, the distance of its image from 
the object-glass, and the distance of the same image from the eye-glass? 
512. Explain and illustrate this proportionality. 513. What other propor- 
tions are remarked concerning astronomical telescopes ? 514. How many 
eye-gla3see bave telescopes which exhibit objects in their natural posi- 
tions? 515. What is a Double Astkonomical Telescope? 



• A telescope of the kind here meant, consisting of four convex lenses, 
is sometimes called a Double Astronomical Telescope. 



TELESCOPES. . 149 

the object-glass by the focal distance of one of the 
eye-glasses. 

CCII. A Galilean Telescope consists of a con- 
vex object-glass and a concave eye-glass, so 
placed that the distance between them is the dif- 
ference of their focal distances.* 

Obs. From a distant object, rays fall upon the convex object 
lens, from which they will proceed towards the focus of that lens, 
But the concave eye-glass renders the converging rays parallel 
when they reach the eye ; whence an image will be formed upon 
the retina. And the pencils of rays being made more diverging 
by passing through the concave lens, the visible image is seen un- 
der a larger angle than the object, and appears magnified. Also, 
because the pencils which form the image only cross each other 
once, the image appears erect. 

CCIII. The Newtonian Telescope consists of 
a tube, towards the end of which a concave mir- 
ror is placed. The converging rays, before they 
reach the focus, are made to fall upon a plane 
mirror placed at an angle of 45°, and thrown 
upwards to the focus of a convex lens fixed in the 
upper side of the telescope, through which the 
eye looks down on the object. 

CCIV. The Gregorian Telescope consists of 
a tube on which a concave mirror, having a hole 
in its centre, is placed. Any parallel rays from 
an object, falling upon this mirror, will, after 
reflection, form an inverted image at its focus. 
This image, however, is intercepted by a smaller 
mirror, which reflects it back to an eye-glass in. 
the hole of the large mirror, through which the 
observer views the object. 

516. What are the construction and principle of the Galilean telescope 1 
Of the Netctonian telescope] Of the Gregorian telescope* And of the 
Herschel telescope '? 

* This kind of telescope derives its name from the great Galileo, who 
has generally been considered the inventor. 

13* 



150 



OPTICS. 



Obs. 1. In the telescopes made by Dr. Herschel, the object is 
reflected by a mirror, as in the Gregorian telescope, and the rays 
arc intercepted by a lens at a proper distance, so that the observer 
has his back to the object, and looks through the lens at the 
mirror. The magnifying power will be the same as in the Newr 
tonian telescope, but there being no second reflector, the brightness 
of the object viewed in the Herschel telescope, is greater than that 
in the Newtonian telescope. 

2. The tube of Dr. Herschel's grand telescope, is 39 feet 4 inches 
in length, 4 feet 10 inches in diameter, every part of which is 
made of iron. The concave surface of the great mirror is 48 inches 
of polished surface in diameter, its thickness 3| inches, and its 
weight is upwards of 2000 pounds. This noble instrument was, in 
all its parts, constructed under the sole direction of Dr. Herschel ; it 
was begun in the year 1785, and completed August 28, 1787, on 
which day was discovered the sixth satellite of Saturn. It magni- 
fies 6000 times.* 

3. Dolland's Achromatic Telescope consists of a compound ob- 
ject-glass, in order to remedy the defect arising from the different 
refrangibility of rays of light. The object-glass consists partly of 
white flint glass, and partly of crown glass, which have different 
refracting powers. These refract in contrary directions ; and the 
excess of refraction in the crown glass is made such as to corres- 
pond to the deficiency of refraction in the flint glass; so that the 
colours are destroyed which would arise in any single object- 
glass. For it must be recollected that the dispersive power of dif- 
ferent mediums is not proportioned to their mean retractive pow- 
er ; and hence, the inventor, Mr. J. Dolland, was enabled, after a 
variety of trials and considerations, to construct such telescopes as 
have been denominated achromatic, that is, colourless ; the object- 
lens of which he compounded of glasses of different refractive and 
dispersive powers, so well proportioned, as not to separate the 
light into its primitive colours; hence the objects, which are seen 
through it, appear of their natural colours. 

4. Dr. Priestly observes, that the easiest method of finding the 
magnifying power of any telescope, by experiment, is to measure 
the diameter of the aperture, of the object-dass, and that of the 
little image of it, which is formed at the place of the eye. An 



517. What are the dimensions of the several parts of Dr. Herschel's 
grand telescope 1 513. What is the principle of the Achromatic tele- 
scope? 

* The frame by which this gigantic telescope was supported, having 
decayed, it was taken down in 1822, the year oi Dr. Herschel's death, and 
an other telescope was erected in its place by his son. The reflector of 
the latter is 18 inches in diameter, and the focal distance 20 feet. An 
other large telescope of tins kind, and perhaps the largest now in exist- 
ence, was erected in 1820 at the Royal Observatory of Greenwich, in 
England. This has a concave reflector of 15 inches in diameter, and 25 
feet focal distance. 



THE CAMERA OBSCURA. 151 

other method, is to observe at what distance any book can be read 
with the naked eye ; and then removing- the book to the greatest 
distance at which it can be distinctly read by the help of the tele- 
scope, the greater distance divided by the less, gives the power 
of tne telescope. 

CCV. The Camera Obscara is made by fix- 
ing a convex glass in a hole of a window shut- 
ter, and, if nolight enters the room but through 
the glass, the pictures or images of all objects on 
the outside may be seen in an inverted position 
on a white paper placed in the focus of the lens. 
If the convex lens be placed in a tube in the in- 
side of a box, within which is a looking-glass 
sloping backwards, it is called a Portable Ca- 
mera Obscura. 

Fig. 53. 

Illus. Fig. 58, represents 
a box consisting- of two parts. 
The outer ABCDEFG has a 
shutter or cover LXPQ. which 




ij-moves round a hinge PQ.. and 
■ when open, as in the figure. 
j -j^ it carries two lateral boards 
GJ- which serve to exclude the 

external light as much as possible from the rough glass O, upon 
which the observer is to look. The foreside of the box is want- 
ing, and in that aperture an other narrower box EHIXG slides; this 
box wants the inner side, and has a convex glass lens fixed at I. 
If this machine be turned with the lens I towards any objects that 
are well illuminated, an inverted picture of them will be formed 
within the box on the side ABCD. and thatpicture may be ren- 
dered distinct by moving the sliding box EH GK in or out, in 
order to adjust the focus according to the distance of the objects. 
At the back part of the box a fiat piece of looking-glass is situ- 
ated at an inclination of half a right-angle, as it is shown by the 
dotted lines BR; in consequence of which the rays of light fall 
upon the looking-glass, and are reflected upwards to the rough 
glass O. The picture then is formed upon that rough glass, and 
will appear erect to a spectator situated behind the box, and look- 
ing down upon the glass O, from which a drawing may be made. 

5x9. What is the easiest method of finding the magnifying power of a 
tel scope 1 520. What is the principle of the Camera Obscura? 521. 
What are its construction, mode of operation, and use 7 



152 optics. 

Obs. From comparing the description of the eye, in Article 178, 
with the preceding description of the Camera Obseura, it will 
readily appear, that the eye is a most excellent camera ohscnra, 
having all the necessary properties of it to a remarkable degree of 
precision. It is a dark room, with one aperture for the admis- 
sion of light, with lenses fit to form a picture of external objects 
on the posterior parts of its cavity, and is capable of all the neces- 
sary adjustments within certain limits. 

CCVI. The Magic Lantern is an instrument 
used for magnifying paintings on glass, and 
throwing their images upon a white screen in a 
darkened chamber ; Figs. 59 and 60. 

And the magic lantern becomes the Phan- 
tasmagoria^ when the glass slides are rendered 
opaque between the objects. 



Fig. 59. 



Fig. 60. 




E 



lllus. Fig. 59 represents the machine with the effect it pro- 
duces. Fig. 60 shows the internalparts of themachine placed at 
their proportionate distances. The lantern contains a candle A, 
a reflector M N, which is so situated as to have the light A in its 
focus. On the fore part of the lantern, there is a thick double- 
convex lens, C D, or a plano-convex usually called a buWs eye, of 
short focus. The lantern is closed on every side, so that no light 
can come out of it, except what passes through the lens C D. In 
the direction of this lens, there is a tube x, Fig. 59, fixed in the 
lantern, which has a lateral aperture from side to side ; through 
this the glass slider aa, with the painted small images, is moved 
in an inverted position. G H, Fig. 60, represents one of these im- 
ages. The fore part of the tube # contains an other sliding tube, 
which carries the double-convex lens E F. The effect of those 
parts is as follows ; — The thick lens C D throws a great deal of 

522. What is remarked concerning the analogy between the camera 
obseura and the eye ? 523. What is the Magic Lantern 7 524. What 
is the principle of its construction and use ? 






ELECTRICITY. 153 

light from the candle A upon the image G H, and to increase that 
light still more, the reflector M N is often, but not always, placed 
in such lanterns ; for, as the flame is in the focus of the reflector, 
the light proceeds in parallel lines from the reflector to the lens 
C D. The image G H being thus well illuminated sends forth rays 
from every point, which, by passing through the lens E F, are con- 
verged to a focus upon the wall, and form the large images as is 
shown in Fig. 59. In some magic lanterns, instead of the single 
lens E F, two lenses are used of less curvature, and set at a little 
distance from each other, and act rather better than a single lens. 
Obs. From comparing the preceding description of the magic 
lantern, with that of the solar microscope, as given in Article 197, 
it will be readily seen that such a microscope is a complete solar 
magic lantern, and that a magic lantern may be constructed so as 
to correspond to any improvement in the construction of the solar 
microscope. 

CCVII. The Multiplying Glass is made by 
grinding down the side of a convex glass into 
several flat or plane surfaces. 

Obs. An object seen through a multiplying glass, will appear 
multiplied into as many different objects, as the glass contains 
plane surfaces. 



ELECTRICITY. 

CCVIII. The Earth, and all the bodies with 
which we are acquainted, are supposed to con- 
tain a certain quantity of an exceedingly elastic 
fluid, which is called the Electric Fluid. 

Obs. 1. The surfaces of all natural bodies are supposed to pos- 
sess the power of exciting or exhibiting this subtile agent, which 
passes through them with more or less facility, according to their 
different powers of conducting it. 

2. The quantity of electric fluid usually belonging to any sur- 
face or to any body, may be called its natural share ; and so long 
as each body contains neither more nor less than this certain quan- 

525. What is the difference of principle between the magic lantern and 
the solar microscope? 526. What is the difference between the magic 
lantern and the Phantasmagoria? 527. What is the Multiplying 
Glass ; and how is it made? 528. What is the Elective Fluid; and in 
what manner does it exist? 529. What parts of bodies possess the power 
of exhibiting the electric fluid internal or external 1 



ELECTRICITY. 

tily, it seems to lie dormant or invisible, and to produce no sensi- 
ble effects. But when any surface or body becomes possessed of 
either more or less than its natural quantity, it is said to be electri- 
Jled, and is then capable of exhibiting- certain peculiar appearances 
which are ascribed to ihepoivcr, called electric. 

Exp. 1. Take a stick of sealing-wax and rub it with the hand, 
or with a piece of flannel, or on a coat sleeve, and it will have the 
power of attracting small bits of paper, or other very light sub- 
si ances, when held near them. 

2. If a clean and dry glass tube be rubbed several times up- 
wards and downwards, and then presented to any small light sub- 
stances, it will immediately attract and repel them alternately for 
a considerable time. The tube is then said to be excited. 

3. If a glass tube be rubbed with apiece of dry silk, several 
times in the dark, and then brought within about half an inch 
of the finger, a lucid spark will be seen between the finger and the 
tube, accompanied by a snapping noise ; arfa the finger at the same 
time will perceive the sensation of a prick from a pin. The attrac- 
tion, repulsion, sparkling, and noise, are the effects of electricity, 
and are denominated electrical appearances . 

The following figure represents the appearance of Sparks of 
Electricity ; 

Fisr. 61. 

4. If an oblong piece of me- 
| taJ, such as a poker, be sus- 
| pended by means of a dry silk 
I string, and the excited glass 
a tube be presented to its upper 
Send, then the lower end of the 
[metallic body will exhibit the 
] same phenomena as the tube 
I itself; which shows that the 
electric fluid passes through the metal. 

5. If, instead of the metallic body, a stick of glass or sealing 
wax, be suspended, none of these phenonema will be exhibited, 
which proves that the electric fluid does not pass through these 
substances. 

6. If we rub a piece of amber with dry fur, or woollen cloth, and 
then hold the amber over anv light substances, such as small 
pieces of paper, or the down of a feather, the light bodies will be 
attracted by the amber. The same effect will be produced by rub- 
bing the glass of a w 7 atch against the sleeve of a coat, and still 
more powerfully by rubbing a glass tumbler in the same manner. 
In the latter case, the glass will immediately attract light sub- 

530. Under what circumstances are the effects of the electric fluid 
shown? 631. What is understood by the Natural Share or equilibrium 
of the electric fluid 1 532. When is a body said to be Electrified ? 
When apiece of sealing-wax, or glass, is rubbed, why does it acquire the 
power of attracting and repelling light bodies ? 







ELECTRICITY. 155 

stances, and give a lucid spark to the finger or to any metallic sub- 
stance brought near it, accompanied by a crackling sound. Many 
other substances may be excited with similar effects; while differ- 
ent bodies of a certain class exhibit no such effects, however ex- 
cited. 

CCIX. The equilibrium of the electric fluid 
could never be disturbed, or, if disturbed would 
be immediately restored, and therefore become 
insensible, were it not that some bodies do not 
admit the passage of the electric fluid through 
their pores, or along their surfaces ; while others 
do ; and hence all known substances are divided 
into two great classes with regard to their pow- 
ers of exhibiting electrical appearances. 

CCX. All those bodies which transmit or con- 
duct electricity from one surface to an other, are 
called Conductors ; and those surfaces that will 
not transmit the electric power, are called Elec- 
trics or Non-conductors. 

Obs. 1. The following substances are non- electrics, or conduct- 
ors of electriciiy, disposed in the order of their degrees of perfec- 
tion ; — gold, silver, copper, platina, brass, iron, tin, mercury, lead, 
other metallic substances, metallic ores, charcoals, plumbago, the 
fluids of animal bodies, water, diluted acids, saline fluids, ice 
above-13° Fahr., snow, living vegetables and animals, flame, or 
the effluvia of flaming bodies ; most saline substances, several 
earthy or stony substances, smoke, steam, rarel>d air, vapours of 
alcobol and ether. 

2. Electrics or non-conductors ; — glass and all vitrifications,even 
those of metals ; all precious stones, all resinous substances, gums, 
amber, sulphur, wax, shell-lac, jet, talc, baked wood, bituminous 
substances, diamond, siik, cotton, feathers, wool, hair, dry paper, 

1>archment, leather, elastic fluids, air, dry vegetable bodies, porce- 
ain, dry marble, camphor, dry chalk, lime, dry metallic oxides, 
oils, ashes of animal and vegetable substances, most hard stones. 

CCXI. When a surface is supposed to have 

534. How is it proved that the electric fluid will pass through a metal 1 
535. How is it proved that electricity will not pass through glass or seal- 
ing-wax! 536. What is said of the disturbance in the equilibrium of the 
electric fluid 7 537. What substances are Conductors, and what are Non- 
conductors of this fluid 1 ? 



156 ELECTRICITY. 

more than its natural quantity of this fluid, it is 
said to be Positively Electrified; when it is sup- 
posed to have less than its natural share, it is 
said to be Negatively Electrified. 

CCXII. When any electrified conductor is 
wholly surrounded by non-conductors, so that the 
electric fluid can not pass from the conductor 
alonof conductors to the Earth, it is said to be 
Insulated. 

Obs. When a body has acquired an additional quantity of elec- 
tric matter; or has lost a part of what i<= considered its natural 
share, and is at the same time surrounded by bodies through which 
the fluid can not pass, the electrified body must necessarily remain 
in that state, being insulated. 

Exp. The human body is a good conductor of electricity ; but if 
a person becomes insulated, by standing on a cake of resin, or on 
a stool supported by glass legs, the electric fluid can not pass from 
him to the Earth, or from the Earth to him ; and if he is touched 
by an other person standing on the ground, he will exhibit sparks. 

CCXIIL The principal method of exciting 
the electric fluid is by contact, pressure, or fric- 
tion. When an electric and non-electric are rub- 
bed against each other, electricity is excited, and 
the electric power passes from the non-electric 
or conductor, to the electric or non-conductor. 

Obs. The electric fluid may be excited by rubbing, by pouring 
a melted electric into an other substance, — by heating and cool- 
ing, and by evaporation. 

Exp. I. If a smooth glass tube be rubbed with the hand, the 
electric fluid will leave the hand, and pass upon the tube, which 
will then have more than its natural quantity. 

2. And if the finger, or any conducting substance, be presented 
to the tube, the electricity will then pass to it. 

Obs. Certain changes in the forms of substances, are always 

539. When a surface is supposed to have more than its nature! quan- 
tity of electricity, what is sain of it? 539. What is observed of a surface 
which has less than its natural quantity of this agent? 540. When is a 
body said to be Insulated? 541. Under What circumstances can the 
human body be made to give an electric shock? 512. What are the 
principal methods of exciting the electric fluid ? 543. By what are 
the effects of the electric fluid distinguished ? 



ELECTRICITY. 157 

connected with electrical effects. Thus, when vapour is formed, 
or condensed, the bodies in contact with the vapour become elec- 
trified. If, for instance, a plate of metal be strongly heated, and a 
drop of water be then poured upon the plate, at the moment the 
water' rises in vapour, the gold leaves of the electrometer will 
diverge with negative electricity. Sulphur, after being melted, 
becomes strongly electrical during the time of congelation. 

CCXXV. The effects of the electric fluid are 
distinguished by an attractive and rejmlsive 
quality, which it communicates to different 
bodies, and which ceases to operate, when the 
bodies acting on each other come in contact, or 
when they are touched by other bodies. 

Obs. Two surfaces, both positively, or both negatively electric 
tied, mechanically repel each other ; and two substances, of which 
one is positively, and the other negatively electrified, mechani- 
cally attract each other. 

CCXV. If any person who is insulated, rubs 
a glass tube, the person and the glass tube will 
become electrified, and be capable of attracting 
and repelling light bodies ; but the electricity of 
the person will differ from that of the tube. 

Exp. 1. Let two cork balls, connected by a linen thread, be 
held by a silk thread, attached to the middle of the former, at some 
distance from a wall ; then bring the excited tube near the balls, 
and it will first appear to attract, and soon after repel them ; this 
apparent repulsion will continue for a considerable time, though 
tne tube be removed. 

2. Let an other pair of cork balls be brought in contact with the 
insulated person, and they will appear to repel each other. 

3. But if the two pair of balls be brought near, they will attract 
each other, and the electrical power will disappear; which shows 
that there are two electricities, one being the reverse of the other, 
and seeming to have what the other wants. 

4. If the insulated person rub a stick of sulphur, or resin, or 
sealing-wax, that substance will acquire the electrity which, in the 
preceding experiment, was acquired by the insulated person. 

544. When do two electrified bodies repel each other ; and when do 
they attract each other 7 545. How is it shown that when an insulated 
person rubs a glass tube, the person is electrified differently from the 
tube 1 

14 



158 ELECTRICITY. 

5. Hence positive and negative electricity have sometimes been 
called vitreous and resinous electricity. 

CCXVI. Opposite electricities always accom- 
pany each other, for if any surface becomes posi- 
tive, the surface with which it is rubbed becomes 
negative ; and if any surface is rendered posi- 
tive, the nearest conducting surface will become 
negative. 

CCXVII. When one side of a metallic or 
other conductor receives the electric fluid, its 
whole surface is instantly pervaded ; whereas, 
when an electric is presented to an electrified 
body, it becomes electrified on a small spot only. 

Obs. There is a stone found in many parts of the world called 
Tourmalin, which is sometimes crystallized as a nine-sided prism, 
terminated by a three-sided and a six-sided pyramid. When 
this substance is gently heated, it becomes electrical, and one ex- 
tremity, that terminated by the six-sided pyramid, is positive, the 
other is negative; to a certain extent, its electricities are exalted 
by increasing the temperature ; when it begins to cool, it is still 
found electrical, but the electricities are changed ; the pyramid, 
before positive, is now negative, and vice versa. When the stone 
is of considerable size, flashes of light may be seen along its sur- 
face. There are other gems and crystallized substances, which 
possess a property similar to that of the tourmalin. The lumi- 
nous appearance of some diamonds, when heated, probably de- 
pends upon their electrical excitation. The substance called bora- 
cite, which is a cube, having its edges and angles defective, be- 
comes electrical by heat, and, in one variety, presents no less than 
eight sides, in different states, four positive, four negative ; and 
the opposite poles are in the direction of the axis of the crystal. 

CCXVIII. Tf to one side of an electric, as a 
pane of glass, we communicate positive electri- 
city, the opposite side will become negatively 
electrified ; and the plate is then said to be 
Charged. 

546. What is meantby Vitreous and Resinous electricity ? 547. When 
the electric fluid is excited by rubbing two surfaces together, are they 
both in the same electrical state? 548. How does the electric fluid ope- 
rate upon conductors and upon electrics? 519. What is stated concern- 
ing the stone called Tourmalin 1 



ELECTRICITY. 159 

Obs. The positive and negative electricities, in the above case, 
can not come together, unless a communication, by means of con- 
ductors, is made between the sides of the glass ; and in like man- 
ner, as a plate of glass is charged, so the plate of air lying between 
any electrified surfaces is always charged. 

CCXIX. When two surfaces oppositely elec- 
trified are united, their powers are destroyed, and 
if their union is made through the human body, 
it produces an affection of the nerves called an 
Electric Slwck. 

Obs. The electric spark thus taken upon any part of a living 
animal body, causes a disagreeable sensation, which is more or less 
so, according as the spark is stronger or weaker, and as the part 
is more or less delicate, or the person more or less sensible. 

CCXX. As the ' exciting of a tube is very 
laborious for the operator, and the electricity 
produced by that means is small in quantity, 
globes and cylinders have been used, in order to 
produce a higher degree of electricity than can 
be effected by a simple tube ; and machines 
have thus been contrived for rubbing together 
the surfaces of electrics and non-electrics, and 
for collecting the electric fluid, when so excited. 

Fig. 62. 

lllus. 1. Fig. 62, represents 
an Electrical Machine. G E 
F is a strong board, which 
supports all the parts of the 
machine, and which may be 
O fastened to a table by means 
of one or more iron, or brass 
clamps, as Q,. The glass cyl- 
inder d B. is supported by the 
two glass legs, G and E. I R 
is the rubber of leather, and 
silken flap. The rubber is 
spread with an amalgam of 
mercury and zinc, or tin. The 
rubber or cushion is fastened 
to a spring, which proceeds 
from a socket cemented on 
the top of the glass pillar SL 




160 ELECTRICITY. 

The lower part of this pillar is fixed into a small board, which 
slides upon the bottom board of the machine, and by means of a 
screw-nut and a slit at H, may be fixed more or less forward, in 
order that the rubber may press more or less upon the cylinder. 
NFis a glass pillar, which is fixed in the bottom board, and sup- 
ports the prime conductor ML of hollow brass, or tin plate, or 
coated wood, which has a collection of pointed wires, at L and 
knobbed wire at M. From the brass knob, O, a longer spark may 
be drawn with the hand than from any other part of the con- 
ductor. 

2. When the cylinder is turned swiftly, the friction of the glass 
against the rubber causes the electric fluid which was upon the 
rubber to pass to the glass, from which it is conveyed to the points 
of the prime conductor, which are presented to every part of the 
cylinder in succession. If one end of the chain be put on the knob 
.r, and the other end hang on the ground, there will then be a con- 
stant supply of the electric fluid to the prime conductor, which 
will be discharged in sparks to any body presented to it. The 
rubber is resupplied by means of the surfaces in immediate con- 
tact, and these again are supplied by the general mass of the fluid 
that is lodged on the Earth. 

Bodies or surfaces, that are charged with the same electricity, 
appear to repel each other ; but if one has more and the other less 
than its share, they will appear to attract each other. 

Exp. 1. If a tuft of feathers be hung on the prime conductor L 
M, Fig 63, the moment they are electrified by turning the wheel 
of the machine, they will endeavour to avoid each other and stand 
erect ; because, being all electrified by the same electricity, they 
repel each other. 

2. A large feather will, if placed in the hole z, when the ma- 
chine is worked, become beautifully turgid, expanding its fibres 
in all directions ; and they collapse, when the electricity is taken 
off, by presenting any conducting substance to them. 

3. Excite a glass tube eighteen or twenty inches long, then pre- 
sent to it a small feather, which will first have the appearance of 
being attracted by it, and afterwards jump from it. If no other 
body happens to be in the way, it will tend towards the ground ; 
but if the tube be held under it, it will be still repelled, and may 
be driven about for a considerable time. 

4. Suspend a plate of metal from the conductor, and underneath 
it, at the distance of about three or four inches, put an other plate 
of the same size ; upon the lower one, small feathers, pieces of 
paper, &c. may be placed ; these will, as soon as the machine is 

550. When two surfaces oppositely electrified are united, what is the 
effect? 551. What is the construction of the Electrical Machine; and 
what are the uses of iis several parte 1 552. What is the law in regard 
to the eliM-tricil attraction and repulsion, between bodies of the same, 
or of a different name, when charged with electricity 1 553. How is 
this principle illustrated by means of experiments 1 



ELECTEICITV* 161 

worked, jump to the plate, from which they will be repelled and 
fly to discharge themselves upon the lower plate, after which they 
will be- attracted and repelled again, and so continue till the elec- 
tricity of the upper plate is completely discharged- 

5. If two balls made of cork, or the pith of elder, about the size 
of large peas, be fastened to silk threads, they will hang parallel 
to each other, and be in contact; but when bro'ught near the elec- 
trified prime conductor, they will strongly repel each other. 

6. These balls, in their electrified state, show whether the elec- 
tricity is positive or negative ; for if it be positive, by applying 
an excited stick of sealing-wax, the thread will collapse ; "but, 
if it be negative, the sealing-wax will make them recede still 
farther. 

CCXXI. An Electrometer is an instrument 
invented to measure the degree of electrification 
of any substance. 

A pair of cork or pith balls, or pieces of gold- 
leaf, are used to discover the presence or strength 
of electricity, and denominated an Electrometer. 

Fig. 63. 

Exp. Fig. 63, represents a mtadrant electrometer, 
which may be affixed in the hole z of the prime con- 
ductor. Fig. 62. It consists of a very light rod, and 
pith ball A" turning on the centre of a semicircular B. 
According to the strength of the electricity the pith 
ball flies up, and the scale marks the degree in which 
the prime conductor is electrified. 

CCXXII. If a surface, containing only its na- 
tural share of electricity, be brought near a body 
that is electrified, positively or negatively, a part 
of the opposite electricity, in the form of a spark, 
will force itself through the air, from the latter to 
the former. 

Obs. The electric fluid, being accumulated on any body, will 
pass to any conductor brought near to the body; when it "passes 

554. What effects will be indicated by two pith or cork balls? 555. 
What is an Electrometer 1 556. What effect takes place, when a body, 
containing only its natural quantity of electricity, is brought near a body 
that is electrified either positively or negatively ] 557. What is ob- 
served concerning the motion of the electric fluid from one body to an 
other 1 

14* 




I 



162 ELECTRICITY. 

from pointed wires, or is received by them, it is conveyed in a con- 
tinued stream ; but when it passes from a surface which has no 
sharp points, or is received by such a surface, it is discharged 
with an instantaneous explosion or spark. 

CCXXIII. When two surfaces, one electrified 
positively, the other negatively, approach each 
other, the superabundant electricity rushes vio- 
lently from one to the other, to restore the equi- 
librium. 

Obs. 1 . It rushes, says Sir Richard Phillips, through the nearest 
point of physical contact, usually some spicula on the surface, and 
in this spicula or point is consequently concentrated the entire 
power of the opposing surfaces. Hence the positive side exhibits 
brushes at the points, diverging, diffusing, and vanishing; and 
the negative, a concentrated and uniting star. 

2. When bodies are negatively electrified, thev receive the fluid 
from other bodies brought near them. 

CCXXIV. If an animal be placed so as to 
form part of this circuit, the electricity, in pass- 
ing through it, produces a sudden and violent 
sensation, called the Electric Shock. 

CCXXV. The motion of electricity, in pass- 
ing from a positive to a negative body, is so 
rapid, that it appears to be, in truth must be, in- 
stantaneous. 

Obs. 1. The writer referred to above, says it is analogous to 
light and shade, and therefore necessarily coincident. 

2. The electric fluid can be conveyed through an insulated con- 
ductor of any length, and its passage from one side of a charged 
body to the other, is apparently instantaneous, through whatever 
length of a metallic, or other good conductor, it is conveyed. 

3. When any electric substance is electrified, it icill continue in 
that state till some conductor conveys away the accumulated or 
restores the deficient fluid ; which will be done more or less rap- 
idly, according to the degree of conducting power in the conductor, 
and the number of points m which it touches the electric. 

CCXXVI. When any part of one side of a glass 

What is the Electric Sliock? 559. Is the motion of the electric 
fluid instantaneous i 



ELECTRICITY. 163 

is presented to a body electrified positively or ne- 
gatively, that side of the glass becomes possessed 
, of the contrary kind of electricity to the side of 
the body it is presented to, and the other side of 
the glass is possessed of the same kind of electri- 
city as the other body. 

Exp. If the knob O of the prime conductor Fig. 62, be electri- 
fied positively, and a pane of s^lass be presented to the side next to 
O, it will be negatively electrified, and the other side will be posi- 
tively electrified. 

CCXXVII. Electricity may be communicated 
to the whole surface of glass, or any part of it, if it 
t>e covered with a metallic substance, as tin-foil ; 
and this is called coating' the glass. 

CCXXVIII. If a glass plane, or cylindrical ves- 
sel, coated on both sides with tin-foil, or any other 
conducting substance, be charged with electri- 
city, that is, positively electrified on one side, and 
consequently negatively electrified on the other ; 
a conducting communication being made from 
one side to the other by some non-electric, the 
plane, or vessel, will be suddenly discharged, 
with an explosion. 

Obs. Glass of any form, provided it be sound, will answer the 
purpose ; but cylindrical jars are chiefly used. 

Exp. Charge a plate of glass, coated with tin-foil, to within 
about an inch and a half from the edge ; and, upon forming a com- 
munication from one side to the other, a sudden discharge or ex- 
plosion will take place. 

CCXXIX. A glass bottle, or jar, properly 

560. Does the electric fluid spread or diffuse itself over the surface of 
glass? 561. How can it be communicated to a part of the surface only ? 
562. What are we to understand by the Coating of glass T 563. What is 
said concerning the charging and "discharging of a glass plane or cylin- 
der'? 564. What happens when a communication is made between both 
sides of a coated and charged glass ? 565. What shaped glasses answer 
"best for such experiments 1 




164 ELECTRICITY. 

coated internally and externally, for electrical 
purposes, is called a Ley den Phial, or Ley den 
Jar, from the name of the city where this pro- 
perty was first discovered. 

Fig. 64. 

Illus. Fig. 64 represents a Leyden jar coat- 
^_A_ed with tin-foil on the inside and outside, with- 
in about three inches of the top of its cylindri- 
cal part, and having- a wire with a round brass 
knob, or ball, A, at its extremity. This wire 
passes through the cork, or wooden stopper, 
I and at its lowest extremity is a piece of chain 
that touches the inside coating in several parts. 
To charge this jar, a communication is made 
between the electrical machine and the brass knob A, while the 
outside of the jar communicates with the Earth by the table or the 
hand. 

The Leyden phial is sometimes called an Electric Jar, and the 
experiment of charging and discharging a coated electric, in gene- 
ral, has been called the Leyden Experiment. 

Exp. 1. Bring the knob A of the jar near the prime conductor, 
and after a few turns of the machine the jar will oe charged ; that 
is, the inside of the jar will be positively, and the outside nega- 
tively electrified ; or if the inside is negatively, the outside will be 
positively electrified. R is a discharging rod, v/hich is used to 
convey the superabundant electricity from one side to the other, 
where there is less than the natural share. The discharging rod 
consists of two brass .knobs ; a a attached to wires, which move 
round a joint x, fixed to a glass handle R. 

2. When one of the knobs is applied to the ball A, and the other 
to the outside coating, a communication is made between the out- 
side and inside of the jar, by which the equilibrium is instantly 
restored by the superabundant electricity passing from one side to 
the other, appearing in the form of a vivid flash, accompanied by 
a loud report. 

3. A shock may be taken by putting one hand to the outside 
coating, as at a, and bringing the other to the knob A. - 

4. Any number of persons may receive the shock together by 
laying hold of each others' hands, the person at one end touching 
the outside of the jar, and the person at the other end bringing his 
hand near the knob A. If there were a hundred persons so situ- 
ated they would every one feel the shock at the same instant. The 

566. What is a Leyden Pitied, or Jar ; and why so called ? 567. What 
are the structure and the method of charging and discharging a Leyden 
jar 7 56S. What is meant hy the Electric Jar, and by the Leyden Expe- 
riment ? 569. What is said about the discovery of the method of accumu- 
lating electricity 1 



ELECTRICITY. 165 

electric fluid may be thus conveyed many miles in a moment of 
time. 

Obs. Although the electricity developed by the friction of a 
glass tube, or a glass cylinder, was sufficiently strong for exhibit- 
ing many interesting experiments, and for the purpose of investi- 
- gating - the nature and properties of the electric fluid, yet it was 
not till the method of accumulating electricity was discovered, 
that philosophers became acquainted with the overpowering en- 
ergy- of this subtile agent. This remarkable property was disco- 
vered by Von Kliest, in 1745, but it was first satisfactorily noticed 
at Leyden, where this great step in the progress of the science was 
made by the celebrated 3Iusschenbroeck, in his invention of the 
Leyden phial or jar. 

The discovery of the effects of electricity, as exhibited by a Ley- 
den phial, immediately drew the attention of all the philosophers 
in Europe. The accounts which some of thenx gave to their friends, 
concerning the experiments, border very much on the ludicrous. 
We are told that 31. 3Iusschenbroeck, who tried the experiment 
with a glass bowl, informed 31. Reaumur, in a letter written soon 
after the experiment, that rue felt himself struck in his arms, shoul- 
ders, and breast, so that he lost riis breath ; and it was two days 
before he recovered from the effects of the blow and the terror. 
He added, that he would not take a second shock for the whole 
kingdom of France. M. Allemand, who made the experiment 
with a common beer glass ; said that he lost his breath for a few 
moments, and then felt such an intense pain all along his right 
arm, that he was apprehensive of bad consequences ; but it soon 
went off without inconvenience. These exaggerated descriptions, 
it is supposed, must have arisen from terror or love of the mar- * 
vellous. 

CCXXX. Several Leyden jars, connected to- 
gether by making a communication between all 
the outsides, and an other communication be- 
tween all their insides, form an Electrical 
Battery. 

lllus. Fig. 65 represents a battery, consisting of sixteen jars, . 
coated with tin-foil, and disposed in a proper box. The wires, 
which proceed from the inside of every four of those jars, are 
screwed, or fastened, to a common horizontal wire E, which is 
knobbed at each extremity, and by means of the wires F F F, 
the inside coating- of 4, S. or 12, or of all the sixteen iars, ma}- be 
cemented. The inside of the box, which contains these jars, is 
lined with tin-foil. On one side of this box there is a hole, through 

570. What constitutes an Electrical Battery 7 571. What remarkable 
experiments maybe performed by means of the electrical battery 1 



166 



ELECTRICITY. 




which a strong wire or hook passes, which communicates with 
the lining of the box, and of course, with the outside coating of the 
jar. To this hook a wire is occasionally fastened, which connects 
it with one branch of the discharging rod, BBCA. 

CCXXXI. By means of the electrical battery, 
electricity is capable of producing the most pow- 
erful effects. 

Exp. 1. A very slender wire, as x, being made part of the cir- 
cuit, will, by the discharge of the battery, instantly become red- 
hot. It sometimes melts into small globules of different sizes. 

2. If, between two slips of window-glass, some gold-leaf be 
placed, and the slips of glass be pressed firmly together, and the 
shock from a battery be sent through them, the gold-leaf will be 
forced into the pores of the glass. 

3. If the gold-leaf be put between cards, and a strong charge be 
passed through them, it will be completely fused. 

4. Gunpowder may also be fired by the electrical battery. 

5. Place apiece of lighted camphor in a metallic spoon, which 
must communicate with the prime conductor; and when the con- 
ductor is electrified, the camphor will be seen to throw out nume- 
rous ramifications, and will shoot forth like a growing vegetable. 

CCXXXII. Metallic points receive or attract 
the electric fluid from bodies silently and in a 
continued stream. 



572. What are the phenomena respecting the electric fluid when issu- 
ing from metallic points 1 



ELECTRICITY. 167 

Obs. 1. When the electric fluid enters at a point, it appears in 
the form of a star ; but when it goes out from a point, it exhibits 
the appearance of a brush. 

2. And a current of air seems to proceed from the sharp points 
of electrified bodies; for, a wive with sharp points, bended in op- 
posite directions, and suspended on the point of a perpendicular 
wire inserted in the conductor, will be carried round by the current 
proceeding from the points. 

3. Delicate apparatus maybe put in motion by the electric fluid, 
when issuing from a point ; and hence we have electrical orreries, 
electrical mills. &c. 

4. The property of the electrical fluid, in being silently attract- 
ed by metallic points, renders such points of a supposed use in 
defending buildings from the effects of Lightning, which is no- 
thing more than electricity exhibited on a grand scale. "This is 
done by fixing a pointed iron rod higher than any part of the build- 
ing, and continuing it, without interruption, to the ground, or the 
nearest water. The electric fluid will, by means of the pointed 
rod, be gradually conveyed from the cloud to the Earth, by a con- 
tinued stream, and thus prevent the effects of a sudden and violent 
explosion. 

Exp. Introduce the blunt end of a wire, furnished with project- 
ing points, into the hole z of the prime conductor ; then, by turn- 
ing the cylinder, the flames of electricity will be seen to fly from 
each point in the form of a star ; and if the wire is made to turn 
round upon its centre, he flames will appear as one continued 
circle of fire. 

CCXXXIII. Ldghtmng is found to be a flash, 
produced by the rapid motion of vast masses of 
electric matter, rushing from one part into an 
other ; and Thunder is the noise or sound pro- 
duced by the rushing torrent of the electric pow- 
er, reverberated among the clouds.* 

Obs. 1. The similarity of lightning and electricity is not to be 
remarked in a few appearances only, but is observable throughout 
all their various effects ; and there is not a single phenomenon ex- 
hibited by the one, which may not be imitated by means of the 

573. How can machinery be put in motion by means of electricity 1 
574. How are buildings defended from the effects of lightning 1 575. 
What is Lightning? 576.- What is Thunder) 577. When, and by 
ichom teas the identity of lightning and electricity discovered 7 



* The remarkable discovery of the identity of lightning and electrcity 
was made by the celebrated t)r. Franklin, in the memorable year 1752. 
Previous to that year, the identity of these two principles had "only been 
suspected by philosophers. 



1C8 ELECTRICITY. 

other. Lightning destroys edifices, animals, trees, &c. It is 
always conveyed through the best conductors in its passage, such 
as metal or water ; but when it is obstructed in its progress by 
electrics, or any imperfect conductors, it rendsand disperses them 
in every direction. Lightning burns combustible bodies; and often 
melts metals and other substances. A stroke of lightning fre- 
quently disturbs the magnetic power, sometimes reversing the 
poles of a magnetic needle ; and gives polarity to ferruginous sub- 
stances. All these effects, as before observed, may be produced 
upon a much smaller scale, by means of artificial electricity. 
But besides the great similarity observable between the effects of 
lightning and those of electricity, what fully proves their identity, 
is, that the matter of lightning may be actually brought down 
from the clouds by means of insulated metallic rods, or of electri- 
cal kites, and with it any known electrical experiment may be 
performed. 

2. As electricity appears to result from the general powers or 
agencies of matter, it is obvious, that it must be* continually exhi^ 
bited in nature, and that a number of important phenomena must 
depend upon its operation. When aqueous vapour is condensed, 
the clouds formed are usually more or less electrical ; and the 
Earth below them being brought into an opposite state, by induc- 
tion, a discharge takes place when the clouds approach within a 
certain distance, constituting lightning ; and the undulation of 
the air, produced by the discharge, is the cause of thunder, which 
is more or less intense, and of longer or shorter duration accord- 
ing to the quantity of air acted upon, and the distance of the place 
where the report is heard from the point of the discharge. 

CCXXXIV. Electricity produces frequent 
and important changes in the state of the atmos- 
phere. As an agent in nature, the electric fire is 
doubtless one of the most diffusive, active, and 
powerful ; but the extent and efficacy of its 
agency are far from being understood. 

Obs. 1. Air, when highly rarefied, as in warm climates and 
seasons, becomes a conductor of electricity ; and as the atmo- 
sphere is the great reservoir of radiant matter, it consequently pre- 
sents magnificent displays of electrical phenomena. These dis- 
plays are most frequent and magnificent when the temperature is 
highest. On these occasions, the air undergoes great changes, 
much heat is evolved or set free, the atmosphere is agitated by 
winds, lightnings are discharged, and the clouds precipitated in 

573. What is said of the similarity of lightning and electricity 1 579. 
What atmospherical changes arise from the influence of electricity 1 



ELECTRICITY. 169 

showers of rain, till the equilibrium of electricity, of caloric, &c. 
being restored, a calm state of the atmosphere at length suceeeds. 

2. To measure the absolute quantity otthe electric fluid, either in 
the atmosphere, or in any other substance, is perhaps impossible ; 
and all we know on this subject is, that the electric fluid pervades 
the atmosphere ; that it appears to be more abundant in the supe- 
rior than in the inferior regions ; that it seems to be the immedi- 
ate bond of connexion between the atmosphere and the water 
suspended in it ; and that by its various operations, most of the 
phenomena of the meteors are occasioned. 

It is the opinion of many celebrated philosophers, that the elec- 
tric fluid pervades all space and belongs to all matter ;— and it is 
farther supposed, that the electric fluid is originally derived from 
the Sun, or from the solar rays ; that it issues from that luminary 
in the pure state of electricity ; that, joining the radiant particles 
of our atmosphere, it becomes light, and uniting with the grosser 
matter, it becomes fire. 

The evaporation of water is attended with an absorption of 
this fluid from the surface of our globe ; and on the contrary, the 
conversion of aqueous vapour into water 4s attended with a de- 
position of this subtile fluid; so that there is a circulation in the 
electric fluid, as there is in water. It descends originally from 
the Sun ; pervades the whole substance of the globe ; and perspir- 
ing, as it were, at every pore, ascends beyond the clouds ; and 
passing the extreme boundaries of our atmosphere, returns to the 
Sun from which it emanated. 

Others have conjectured, that beyond the bounds of our atmo- 
sphere, there is a great accumulation of electric matter and hydro- 
gen gas ; and that the origin of lightning, aurora borealis" and 
other luminous meteors, ought to be referred to that region, as the 
great source of electrical phenomena. 

3. The appearance of the Aurora Borealis, or Northei'n Lights, 
has been ascribed, by some, to the agency of the electric fluid ; on 
the supposition that the electric matter of the polar regions passes 
through highly rarefied air, and thus constitutes these singular and 
beautiful phenomena. For, the coruscations of the aurora bore- 
alis, and australis, precisely resemble strong artificial electricity, 
discharged through rare air ; and as the polar regions are non- 
conductors, being coated with ice or snow, and as vapour must be 
constantly formed in the atmosphere above them, the idea of 
Franklin is not improbable, that the auroras may arise from a dis- 
charge of electricity, accumulated in the atmosphere near the 
poles, into its rarer "parts ; though other solutions of the pheno- 
mena may be given on the idea, that the Earth itself is endowed 
with electrical polarity; or that the motions of the atmosphere pro- 
duce the effect. 

■ 580. What is supposed to be the origin of the electric fluid 1 5S1. In 
< what manner has the appearance of the Aurora Borealis been explained 1 

15 



170 galvanism; or, 

4. Most of the great convulsions of nature, such as earthquakes, 
whirlwinds, water-spouts, tempests, hurricanes, &c. and indeed, 
almost all natural phenomena, are generally accompanied by elec- 
tricity, and are often dependant upon the influence of that power. 

Tims, the water-snout is supposed by some to be the result of the 
operation of a weakly electrical cloud, at an inconsiderable eleva- 
tion above the sea, brought into an opposite state ; and the attrac- 
tion of the lower part of the cloud, for the surface of the water, may 
be the immediate cause of this extraordinary phenomenon. 

GALVANIC ELECTRICITY. 

CCXXXIV. Galvanism, or Galvanic Elec- 
tricity, may be defined a series of electrical phe- 
nomena, in which the electricity is developed 
by friction or mechanical means, and in which 
a chemical action is observed to take place among 
some of the bodies employed. 

Obs. 1 . Galvanism derives its name and origin from the ex- 
periments on animal irritability made by Gal v an i towards the end 
of the last century. Galvanism is in fact only an other mode of 
exciting electricity. In ordinary electricity, the effects are ex- 
cited or produced principally by mechanical action ; but the effects 
of Galvanism are produced by the chemical action of bodies upon 
each other. 

2. In the year 1791, Dr. Galvani, of Bologna, discovered that a 
dead frog may have its muscles brought into action by very small 
quantities of electricity. He also discovered that the same mo- 
tions may be produced in the dead animal, merely by making a 
communication between the nerves and the muscles, by means of 
conducting substances. 

3. Some fishes have the property of giving shocks analogous to 
those of artificial electricity ; namely, the torpedo, the gymnotus 
elcctricus, and the silurus electricus. 

If the torpedo, whilst standing in water, but not insulated, be 
touched with one hand, it generally communicates a trembling 
motion or slight shock to the hand. If the torpedo is touched 
with both hands at the same time, one hand being applied to its 
under and the other to its upper surface, a shock will be received 
exactly like that occasioned by the Leyden phial. 

The shock given by the torpedo when in air, is aboutfour times as 

What is the supposed cause of the water-spout 7 583. What is 
meant by Galvanism, or Galvanic Electricity 1 531. Whence does the 
science of Galvanism derive its name and origin ? 585. What is the dif- 
ference between Galvanism and ordinary electricity ? 586. When and 
by whom was the Galvanic principle discovered? 



GALVANIC ELECTRICITY. 171 

strong as when in water ; and when the animal is touched on 
both surfaces by the same hand, the thumb being applied to one 
surface, and the middle finger to the opposite, the shock is felt 
much stronger than when the circuit is formed by both hands. 

The gymnotus electricus, or electrical eel, possesses all the 
electric properties of the torpedo, but in a superior degree. When 
small fish are put into the water wherein the gymnotus is kept, 
they are generally stunned or killed by the shock, and then they 
are swallowed, if the animal is hungry. 

The strongest shock of the gymnotus will pass a very short in- 
terruption of continuity in the circuit. When the interruption is 
formed by the incision^made by a penknife on a slip of tin-foil that 
is pasted on glass, and that slip is put into the circuit, the shock 
in passing through that interruption, will show a small but vivid 
spark, plainly to be seen in a dark room. The gymnotus seems 
also to be possessed of a sort of new sense, by which he knows 
whether the bodies presented to him are conductors or not. This 
fact has been ascertained by a great number of experiments. 

Exp. 1. If a living frog, or a live fish, as a flounder, having a 
slip of tin-foil pasted on its back, be placed upon a piece of zinc, 
whenever a communication is formed between the zinc and tin- 
foil, the spasms of the muscles are excited. 

2. The simplest and most satisfactory method, perhaps, of per- 
forming the experiment on a frog, is to skin the legs of a frosr re- 
cently dead, and to leave them attached to a small part of the 
spine, but detached from the rest of the body, which may also be 
skinned and deprived of the entrails, so as to lay bare the crural 
nerves. La}' this preparation upon a piece of zinc ; then apply a 
piece of silver to trie nerves, and bring the two metals into con- 
tact; the spasmodic contractions will then be so considerable as 

587. What effect does it have on dead animals? 5 C S. What animals 
have the power of giving electric shocks ) 5S9. What is the experiment 
with a living frog, or fish ? 590. What is the simplest and most satis- 
factory method of performing the experiment? 

*In trying the experiments of M. Humboldt on an electrical eel which 
was recently brought to Paris from America, a most singular occurrence 
took place. Several naturalists had already subjected themselves to 
electric shocks, more or less violent, by touching the fish, which was of 
the size of a large eel, when Dr. Yanin'de Saint Yusk seized it with both 
his hands, and was rewarded with a succession of shocks exceedingly 
severe and painful. Indeed he was exposed to real damrer. in conse- 
quence of finding it impossible to loose his hold of the animal, notwith- 
standing its every motion agitated his whole frame to an excessive de- 
gree. An involuntary contraction forced him to grasp it with superna- 
tural strength, and the more he grasped, the more dreadful did the elec- 
trical shocks become. They extorted from him the most agonizing 
cries, which alarmed all present, who were even afraid for his life, as it 
is probable had he continued long in that situation, that death must have 
ensued. Happily, it occurred to him, to replunge the eel into its tube ; 
and scarcely were his hands wetted, when the contact of the water, act- 
ing as a conductor, enabled him to let his enemy slip. 



172 galvanism; or, 

sometimes to displace the metals entirely ; and these convulsions 
will be repeated for some time, as often as the contact is renewed. 

3. If a person place a piece of one metal, as a half-crown, above, 
and a piece of some other metal, as zinc, below his tongue, by- 
bringing the outer edges of these pieces in contact, he will per- 
ceive a peculiar taste, and in the dark will see a flash of light. 

4. If a person in a dark place put a slip of tin-foil upon the bulb 
of one of his eyes, and a piece of silver in his mouth, by caus- 
ing these pieces to communicate, a faint flash will appear before 
his eyes. 

5. Place a silver tea-spoon as high as possible between the 
gums and the upper lip, and a bar of zinc similarly between the un- 
der lip and gums. On bringing the extremities of the two metals 
into contact, a very vivid sensation, resembling a flash of light, 
will be perceived. It is not a little remarkable that this light is 
equally vivid in the dark with the strongest light, and whether 
the eyes are shut or open. 

CCXXXV. The nerves and muscles of ani- 
mals are the most easily affected by the galvanic 
fluid ; but, when exhibited in what is called the 
Voltaic Battery, this kind of electricity possesses 
the most astonishing powers of chemical decom- 
position. 

CCXXXVT. The conductors of the galvanic 
fluid, differ from each other in their conducting 
power. They are divided into two classes, called 
Perfect and Imperfect conductors. 

The Perfect conductors consist of metallic 
substances generally, and charcoal ; and these 
are termed conductors of the First class. The 
conductors of the Second class, or Imperfect 
conductors, are water, and oxidated fluids, as the 
acids, and all substances that contain these 
fluids. 

CCXXXVII. The simplest galvanic combina- 

591. What, parts of animals /ire most easily affected by the galvanic . 
fluid 1 592. In what respect do the galvanic conductors-differ from each 
other; and how are they divided 1 593. What are the Perfect and what 
the Imperfect conductors ? 



GALVANIC ELECTRICITY. 173 

tions must consist of three different conductors, 
not wholly of one class. 

When two of the three bodies are of the first 
class, the combination is said to be of the first 
order ; when otherwise, it is said to be of the 
second order. 

CCXXXVIII. It seems to be indispensably 
requisite, in simple galvanic circles, that the con- 
ductors of one class shall have some chemical 
action upon those of the other. 

Exp. If a piece of zinc be laid upon a piece of copper, and upon 
the copper a piece of card or flannel, moistened with a solution 
of salt water, a circle of the first class is formed ; and then if three 
.other pieces be laid on these in the same order, and repeated 
several times, the whole will form a pile or battery of the Jirst 
order. 

CGXXXIX. When the three bodies which 
form a galvanic circle of the first order, are laid 
on each other, the upper and the under one not 
touching, these two extremes form opposite elec- 
tric states. 

CCXL. The galvanic effects may be increased 
to almost any degree, by a repetition of the same 
simple galvanic combinations, and these repeated 
combinations are called Galvanic Piles or Bat- 
teries, which mav be constructed of various 
forms. 

Exp. 1, Take a number, as for instance, twelve plates of silver, 
and trie same number of pieces of zinc, and also of woollen cloth, 
the last having been soaked in a solution of sal ammoniac in 
water ; with these a pile is to be formed, that is, a piece of silver, 

594. What are the most simple galvanic combinations? 595. When 
is the combination of the First, and when of the Second order? 5P6 
What seems to be absolutely necessary in simple galvanic circles ? 597. 
Bo the conductors of one class have any chemical action on those of the 
other? 598. How is a battery of the first order constructed? 599. 
What parts of a pile are in opposite electric states ? 6C0. In what man- 
aer can galvanic effects be increased ? 
15* 



174 



galvanism; or, 



a piece of zinc, a piece of cloth, and thus repeated. These are 
to be supported by three glass rods placed perpendicularly with 
pieces of wood at the top and bottom, and the pile is complete, 
and will afford a constant current of electric fluid, through any 
conducting substance ; thus, if one hand be applied to the lower 
plate, and^the other to the upper one, a shock will be felt, which 
is repeated as often as the contact is renewed. But the plates 
will soon become oxidated, and require cleaning in order to make 
them act. 

2. An other battery consists of a row of glasses of any shape, 
containing a solution of salt and water; into each of these, ex- 
cept the two on the outside, is put a plate of zinc and an other of 
silver ; these plates communicate by means of the wires, and are 
so fastened that the silver in one glass is connected with the zinc 
in the other ; — when one hand is dipped into the first glass, and 
an other in the last, a shock is felt. The glasses may be of any 
number. 

Fig. 66. 

3. The most convenient 
kind of battery consists of 
a trough B, Fig. 66, made 
of baked wood; three inches 
broad, and about as deep; 
in the sides of the trough 
are grooves opposite to each 
other ; into each pair of 
grooves is fixed by cement 
a plate of zinc and silver soldered together, and in the order of 
silver and zinc ; the cement must be rilled in so as to prevent any 
communication between the different cells. The cells are to be 
filled with water or with a solution of water and nitrous acid, 
when, a communication being made between the first and last 
cell, by means of the hands, a strong shock is felt, and will be 
repeated as often as the contact is renewed. 

4. Several persons, by joining hands, having first wetted them 
with water, may receive the shock. 

5. If plates of copper and zinc, two or three inches square, and 
pieces of cloth of the same size, soaked in a solution of salts or 
sal ammoniac, or nitre, be arranged in the order of copper, zinc, 
moistened cloth, and so on, and made into an insulated pile, of 
which the series are two hundred, several remarkable phenomena 
will occur. When one hand is applied to the bottom of the pile, 
and the other to the top, both hands being moistened, a shock will 
be perceived. When a metallic wire, having a bit of well burned 
charcoal at its extremity, is made to connect the two extremities 




601. What is the simplest mode of constructing a galvanic apparatus, 
or battery 1 602. How is the most convenient kind of battery constructed? 
603. What fluid is used to fill the cells of a galvanic trough. 



GALVANIC ELECTRICITY. 175 

of the pile, a spark will be perceived, or the point of the charcoal 
will become ignited. A wire connected with the top of the pile, 
brought in contact with a sensible electrometer, w r ill cause the 
leaves to diverge ; and, by removing the wire and applying excited 
glass to the electrometer, it will be found that the electricity is 
positive ; a wire connected with the bottom of the pile will affect 
it with negative electricity ; a wire from the middle of the pile 
will have no influence on the instrument. If wires of platina 
from the extremities of the pile be introduced into w T ater, or into 
two portions of water connected by moist substances, oxygen gas 
will separate at the wire exhibiting the positive electricity, and 
hydrogen gas at the wire exhibiting the negative electricity; and 
tHe proportions are such, w T hen the proper circumstances exist, 
that they will produce water when exploded by the electrical 
spark, that is, the volume of hydrogen will be to that of oxygen 
as two to one. If the same wires be introduced into a strong solu- 
tion of sulphuric or phosphoric acid, or into metallic solutions, 
oxygen will separate at the positive surface, the inflammable, or 
metallic matter contained in the solution, at the negative surface. 

CCXLI. The spark from a powerful galvanic 
battery acts upon and inflames gunpowder, 
charcoal, cotton, and other inflammable bodies, 
melts all metals, disperses diamonds, &c. 

Exp. 1. Fill the battery, Fig. 66, with water and nitrous acid, 
in the proportion of nine parts of water and one of acid, and wipe 
the edges of the plates very dry; then the wires w 10 are to be 
fastened to pieces of copper, and" pat into the outer cells ; a a are 
little glass tubes to hold the wires by. Bring the ends of the 
wires together on the plate of glass x, and a spark will be per- 
ceived ; — if gunpowder be laid on the glass between the points of 
the wires, it will be exploded. 

2. Gold and silver leaf may be inflamed in this way ; Dutch 
gold burns with a beautiful green light ; silver with pale blue • 
gold with yellow light. 

3. The most powerful combination that exists, in which the 
greatest number of alterations is combined with extent of surface 
is that in the laboratory of the Royal Institution. It consists of 
two hundred instruments, connected together in regular order 
each composed often double plates arranged in cells of porcelain' 
and containing in each plate 32 square inches ; so that the whole 
number of double plates is 2000, and the whole surface 128,000 
square inches. This battery, when the cells are filled with sixty 

604. What substances can be inflamed by the galvanic spark? 605. 
What experiments maybe referred to, in illustrating the calorific effects 
of the galvanic fluid? 606. What is observed concerning the galvanic 
experiments in the laboratory of the Royal Institution. 



176 galvanism; or, 

parts of water, mixed with one part of nitric acid, and one part of 
sulphuric acid, affords a series of brilliant and impressive effects.. 
When pieces of charcoal about an inch long and one-sixth of an 
inch in diameter, are brought near each other, within the thirtieth 
or fortieth part of an inch, a bright spark is produced, and more 
than half i he volume of the charcoal becomes ignited to whiteness, 
and by withdrawing the points from each other a constant dis- 
charge takes place through the heated air, in a space equal at least 
to four inches, producing a most brilliant ascending arch of light, 
broad, and conical in form in the middle. When any substance is 
introduced into this arch, it instantly becomes ignited ; platina 
melts as readily in it as wax in the name of a common candle ; 
quartz, the sapphire, magnesia, lime, all enter into fusion ; — frag- 
ments of diamond, and points of charcoal and plumbago, rapidly 
disappear and seem to evaporate in it. Such are the decompos- 
ing powers of electricity, that not even insoluble compounds are 
capable of resisting their energy ; for glass, sulphate of baryta, 
fiuor spar, &c. when moistened and placed in contact with elec- 
trified surfaces from the voltaic apparatus, are slowly acted upon, 
and the alkaline, earthy, or acid matter carried to the poles in the 
common order. Not even the most solid aggregates, nor the firm- 
est compounds, are capable of resisting this mode of attack ; its 
operation is slow, but the results are certain ; and sooner or later, 
by means of it, bodies are resolved into simpler forms of matter. — 
Davy. 

CCXLII. By galvanism, many facts in com- 
mon life are explained, which were unintelligible 
before. 

Exam. 1. Porter is said to have a peculiar taste when drank 
out of a pewter vessel ; — here is a complete galvanic circle of the 
second order ; the moisture of the under lip is one conductor of 
the second class, the porter is the other, and the pewter is the 
conductor of the first class. 

2. An other galvanic circle is seen by the discolouration of a sil- 
ver spoon in eating eggs ; the saliva and fluid egg are conductors 
of the second class, and the silver of the first. 

3. Pure mercury retains its splendour a long time ; but let it be 
amalgamated with tin, and it is quickly oxidated. 

4. Works of metal, the parts of which are soldered together 
soon tarnish in the places where the metals are joined. 

5. The Etruscan inscriptions, engraven upon pure lead, are pre- 
served to this day; whereas, some models of lead and tin, of no 
great antiquitv, arc much corroded. 

6. The nails and'thc copper in the sheathing of ships are soon 
corroded about the place of contact. These are the effects of 
galvanism. 

607. What facts in common life are explained by galvanism 1 



GALVANIC ELECTRICITY. 177 

CCXLIII. The powers of the galvanic appa- 
ratus, or effects of galvanism, in producing defla- 
gration on metallic bodies, are greatly increased 
by using plates of a large size ; while, by the 
same means the electrical influence is diminished. 
But, on the contrary, by multiplying the pairs of 
small plates, the electrical influence is increased, 
while the calorific power is diminished. 

Obs. 1. The shock of a batten r containing 80 or 1013 pairs of 
plates, of three or four inches in diameter, is such as few persons 
would be willing to bear more than once. At the same time, such 
a battery produces but feeble effects when discharged through a 
metallic wire. On the contrary, if one or two pairs of pJates con- 
taining the same extent of surface be used, the sensation it gives 
is hardly to be felt, while it will deflagrate a metallic wire of con- 
siderable size. 

2. Professor Hare, of Philadelphia, has^invented a new method 
of extricating the galvanic influence, by so connecting the plates, 
that in effect, only two great surfaces of the metals are presented 
to each other ; exhibiting, virtually, only two galvanic pairs of 
enormous size. By this arrangement, the galvanic action on dif- 
ferent substances, has presented several new phenomena. The 
calorific influence, or power of deflagration, is immensely in- 
creased, while the electrical influence is scarcely to be discovered. 
The Professor has named this new apparatus Calorimotor, or 
heat-mover. 

Such is the galvanic apparatus invented by Professor Hare ; 
and in consequence of his discoveries and the phenomena exhi- 
bited by the calorimotor, the original theory of Galvanism has 
experienced a radical change ; and the inventor of the apparatus 
has offered a new theory of Galvanism, which seems to form a 
new era in t*he history of the science.* 

The new views which he has been induced to offer, seem to be 

confirmed by the operation of the calorimotor — that the galvanic 

^influence arises from a combination of the electric fluid and calo- 

'ric. According to his theory, " Caloric permeates all matter, 

more or less, though with different degrees of facility ; radiates 

through air with immeasurable celerity, distributing itself in the 

60S. In what respect is the galvanic power increased by multiplying 
the pairs of small plates ; and in what respect, by using plates of a large 
size? 609. What Is Professor Hare's invention ? 610. Wliat is the Ca- 
lorimotor ; and why so called. 

* See Sillinian's American Journal of Science and Aits ; also Hare's 
edition of Henry's Chemistry, 



178 galvanism; or, 

interior of bodies, or communicat ins? a reciprocally repellent power 
to atoms, but not to masses. Electricity does not radiate in or 
through airy matter; and while it pervades some bodies, as metals, 
with inconceivable velocity, by other bodies it is so far from being 
conducted, that it can only pass through them by a fracture or 
perforation. Distributing" itself over surfaces only, it causes 
repulsion between masses, but not between the particles of the 
same mass." 

He farther observes — cc The contact of dissimilar metals, when 
subjected to the action of solvents, causes a movement in caloric, 
as well as in the electric fluid ; and the phenomena of galvanism, 
the unlimited evolution of heat by friction, the extrication of gaseous 
matter without the production of cold, might all be explained by 
supposing a combination "between the fluids of heat and electricity." 

3. An other singular discovery respecting the voltaic pile or 
battery, was lately made by a Danish philosopher, Professor Oer- 
sted, of Copenhagen. His famous discovery, which was made 
in the winter of the year 1819, relates to the influence of the 
galvanic fluid on magnetism, and has formed the basis of a new 
branch of science, called Electro -magnetism. 

The principal fact observed by Professor Oersted is this ; — 
When a galvanic battery is so placed as to have its negative pole 
directed towards the south, and the other towards the north, a 
communication being at the same time established over the bat- 
tery, between its two poles, by means of metallic wires ; a mag- 
netic needle placed near the wire will be influenced so as to deviate 
from its natural position, and assume a new one, the direction of 
which depends upon the mode of conducting the experiment. 
When the needle is suspended just above the connecting wire, 
and in a parallel direction, it will immediately move round upon 
its pivot, its northern extremity directing itself towards the West, 
more or less, according to the energy of the battery ; while, on the 
contrary, when the needle is placed below the conducting wire, it 
likewise turns round, but its north pole will in this case point 
towards the East When the needle is on the same horizontal 
plane with the wire, no inclination whatever takes place ; but the 
needle shows a disposition to move in a vertical direction, the pole 
next to the negative side of the battery being depressed when the 
needle is to the east of the wire, and elevated when it is placed on 
the west side. 

Many interesting discoveries have since been made concerning 
this subject, by different philosophers,— M. M. Ampere, Arago, 
and Biotj of Paris, and Sir Humphrey Davy, and Mr. Faraday, 
in England ; but the limits of this work do not admit of any fur- 
ther elucidation of them. 

611. What is Professor Hare's theory of Galvanism 1 612. What 
remarkable discovery in Galvanism has recently been made by 
the Punish philosopher, Mr, Oersted? 013. What is the principal fact 
observed by Mr. Oersted ? 



GALVANIC ELECTRICITY. 179 

CCXLIV. It has been found that there is a 
remarkable analogy between the galvanic influ- 
ence and the nervous influence. The galvanic 
influence being capable, in some instances, of 
supplying that of the nerves. 

Hence it has been conjectured by physiologists, that electricity 
may be concerned in some of the most mysterious phenomena of 
vitality ; and that animals and plants are continually subject in 
the influence of this active and subtile agent. 

Obs. 1. Dr. Wilson Phillip, in his inquiry into the laws of the 
vital functions, has shown that if a nerve be divided, and a stream 
of galvanism be directed along and through the part, whose func- 
tions depend on this, that the function was performed as usual. 
Thus, on dividing the nerves which are distributed to the stomach, 
the process of digestion ceases, though the animal continues to 
live for some time. But, on supplying the place of the nervous 
power by the galvanic influence, digestion was performed as usual. 

2. In the same way, it was found, that when the nerves distri- 
buted to other parts were divided, and the part became palsied for 
want of the nervous power, a stream of galvanism rightly directed 
would again in a good degree restore the action of that part. 

3. Dr. Phillip having conjectured that the heat of animals de- 
pended on the influence of the nerves, wished to observe how far 
the galvanic power might produce this effect. For this purpose, 
some blood was drawn from two animals of the same kind and 
temperature, into two small cups. The blood in one of the cups 
was submitted to the galvanic influence, while the other was 
placed under the same circumstances, except in this respect. 
Now if the galvanized blood remained warm the longest, or had 
its temperature increased, it would show an additional analogy 
between the galvanic and nervous energy, if indeed animal heat 
depends on the latter. The experiment gave a decided proof that 
the galvanic power had some influence on the temperature of 
newly drawn blood, for that portion which was submitted to its 
action, not only remained warm longer than the other, but its tem- 
perature was actually raised several degrees. 

4. From the difficulty m breathing, which animals experience 
on depriving the lungs of a portion of their nervous influence, Dr. 
Phillip was led to make trial of galvanism in asthma, suspecting 
in this disease there might be a w T ant of nervous power. The 
inference which he draws from a very considerable number of 
trials on persons afflicted with this disease, is, that there is a dif- 
ficult transmission of the nervous influence, through the nerves 

614. What analogy is remarked between the galvanic influence and 
the nervous influence'? 615. What effect does galvanism have on warm 
blood I 



180 GALVANISM. 

which supply the organs of respiration, and that in a great ma- 
jority of cases the asthma may be permanently cured, or at least 
relieved by galvanism. 

CCXLV. Galvanism has a peculiar and most 
surprising" effect on the muscles of dead animals, 
their limbs being thrown into violent motion by 
it a considerable time after the life of the animal 
is extinct. 

5. The most striking: effects of galvanism on the human frame 
after death, were exhibited at Glasgow a few years since. 

The subject on which these experiments were made, was the 
body of the murderer Clydesdale, who was hanged at that city. 
He was suspended an hour, and the first experiment was made 
in about ten minutes after he was cut down. The galvanic battery 
consisted of 270 pairs of four inch plates. 

The subject was prepared for the first experiment by making an 
incision into the nape of the neck, and removing a part of the atlas 
vertebre, so as to bring the spinal marrow into view ; at the same 
time an other incision was made in the left hip, so as to lay above 
the sciatic nerve, and an other small one in the heel. The pointed 
rod connected with one end of the battery was now made to touch 
the spinal marrow, while the end of the other was placed in 
contact with the sciatic nerve. Every muscle of the body was 
immediately agitated with convulsive movements, resembling a 
violent shuddering from cold. On moving the rod to the heel, 
the knee being previously bent, the leg was thrown out with such 
violence, as nearly to overturn one of the assistants, who in vain 
attempted to prevent its extension. 

The next experiment was made by directing the galvanic power 
in the course of the phrenic nerve, which goes to the principal 
muscle of respiration, the diaphragm. The effects were far more 
striking than before. " Full, nay laborious breathing," says Dr. 
Ure, "instantly commenced. The chest heaved and fell; the 
abdomen was protruded, and again collapsed with the relaxing 
and retiring diaphragm." 

In the judgment of many scientific gentlemen who witnessed 
the scene, this respiratory experiment was, perhaps, the most 
striking ever made with a philosophical apparatus. The next 
experiment was made by applying one of the wires to the supra 
orbital nerve under the eyebrow, and the other to the heel. Most 
extraordinary grimaces were made ; " Every muscle in the coun- 
tenance was simultaneously thrown into fearful action ; rage, 
horror, despair, anguish, and ghastly smiles, united their hideous 

616. What, is said of the effects of galvanism in the case of asthma % 
617. What effect has it on the muscles of dead animals'? 



MAGNETISM; lSl 

Expressions in the murderer's face." At this period, several of 
the spectators were forced to leave the room from terror oj sick- 
ness, and one gentleman fainted. " In the last experiment, one of 
the wires was made to touch the spinal marrow at the nape of the 
neck, and the other an incision in the top of the forefinger, the fist 
being previously clenched. The finger extended instantly, and 
from the convulsive agitation of the arm, the finder seemed to 
point out the different spectators, some of whom thought he had 
really come to life."' 



MAGNETISM. 

CCXLVL The science of Magnetism ex- 
plains the properties of the load-stone, or natural 
magnet, which is a dark-coloured and hard 
mineral body, and is ascertained to be an ore of 
iron, being generally found in iron mines. 

Obs. The magnetic properties of a natural magnet may be 
communicated to some other substances, which are then called 
Artificial Magnets. These properties can not, however, be com- 
municated to any other substance than iron, nickel, and cobalt. 

CCXLYII. The following are the distinguish- 
ing and characteristic properties of a magnet ; — 

1. The magnet attracts iron and steel. 

2. A magnet, if left at liberty, will point to- 
wards the poles of the Earth, or very nearly so, 
and each end always points uniformly to the 
same pole. 

3. When the north pole of one magnet is pre- 
sented within a certain distance to the south pole 
of an other, they will attract each other. But if 
a north pole of one be presented to the north pole 

618. What is said of the effects of galvanism on the body of the mur- 
derer Clydesdale 1 619. What is-the object of the science of JIagnet- 
iy?n? 620. What is the Natural Magyiet ; and where is it generally 
found? 621. To what substances can magnetic properties be commu- 
nicated 1 622. What are the distinguishing and characteristic properties 
of a magnet 7 What substances are susceptible of being attracted by a 
magnet 1 

IS 



182 MAGNETISM. 

of an other, or a south to a southfthey will repel 
eacli other. 

4. The two poles of a magnet, left at liberty, 
do not lie in the same horizontal direction ; ond 
of them inclines towards the horizon, and, of 
course, the inclination of this causes an elevation 
of the other pole above it. This is called the 
Inclination or Dipping of the magnet. 

5. Any magnet may be made to impart those 
properties to iron or steel. 

Obs. I. All natural and artificial magnets, as well as the sub- 
stances upon which they act, are either pure iron, nickel, and 
cobalt, or such compounds as contain these metals. All magnets 
will attract iron and nickel. 

2. The action and re-action of the magnetic power are mutual 
and equal ; for when a piece of iron, or steel, or other ferruginous 
substance, is brought within a certain distance of one of the poles 
of a magnet, it is attracted by it, so as to adhere to the magnet, 
and not suffer itself to be separated without an evident effort. 

3. The attraction is mutual, for the iron attracts the magnet as 
much as the magnet attracts the iron ; since if they be placed on 
pieces of wood or cork, so as to float upon ihe surface of water, 
it will be found that the iron advances towards the magnet as 

"well as the magnet advances towards the iron ; or, if the iron be 
kept steady, then the magnet will move towards it.* 

CCXLVIII. When a magnet is at liberty to 
move itself freely, it constantly turns the same 
end towards the north pole; and, of course, 
the opposite part towards the south pole of the 
Earth. 

CCXLTX. Those parts of the magnet's surface 

623. What is said respecting the action and re-action, or mutual at- 
traction of two magnets 1 621. What direction does a magnet assume 
when left at liberty to move freely 1 

* The experiment here referred to is very far from proving that the 
attraction is mutual, or that the iron attracts the magnet as much as the 
magnet attracts the iron. The iron must necessarily move towards the 
magnet when the latter attracts it with sufficient power; not it does not 
follow from this, thai the Iron attracts the magnet. However, ivhen a 
piece of iron ed sufficiently near to a magnet, it becomes itself 

a magnet, and in that case may attract that to which it is presented. 



MAGNETISM. 183 

which it turns towards the poles of the Earth, 
are called the North and South Poles of the 
Magnet. 

CCLu The property of pointing to these poles 
is called its Directioe Power; and when it moves 
to place itself in that direction, it is said to Tra- 
verse. 

CCLI. The Magnetic Meridianpasses through 
the poles of the magnet when standing in their 
natural direction. The Declination of the mag- 
net, or of the magnetic needle, is the angle which 
the magnetic meridian makes with the meridian 
of the "place. 

CCLIL Two mag-nets, having a free motion, 
will attract, when different poles are directed to- 
wards each other ; and repel, when the adjacent 
poles are of the same name. Or, in other words, 
the north pole of one magnet will attract the 
south pole of an other; but the two north poles 
or the two south poles of two magnets, repel each 
other. 

Exp. Place a magnetic needle upon a pin stuck on a table, and 
when it stands steady, place an iron bar, eight inches long, and 
half an inch thick, upon the table, so that one end of it may be on 
one side of the north pole of the needle, and near enough to draw- 
it a little out of its natural direction. In this situation approach 
gradually the north pole of a magnet to the other extremity of 
the bar, and it will be seen that the needle's north end will re- 
cede from the bar, in proportion as the magnet is brought nearer 
to the bar. 

CCLIII. A needle, before it is rendered mag- 
netical, rests on its centre parallel to the plane of 
the horizon ; but as soon as it is made to receive 

625. What are the Poles of a magnet ? 6-6. What is the Magnetic 
Meridian! 627. What is meant by the Declination of the magnetic 
needle 1 628. Which of the magnetic poles are attractive of each other j 
and which are repellent? 



184 MAGNETISM. 

the magnetic power, it will incline towards the 
Earth, elevating one of its poles above and the 
other below the horizontal line passing through 
its centre. Tins property is called the Inclina- 
tion or Dipping of the magnetic needle, 

CCLIV. The magnetic power may, by proper 
methods, be communicated from a natural mag- 
net, and from one piece of iron to an other, which 
then becomes an artificial magnet ; and this com- 
munication of power is without any apparent loss 
of power in the magnet. 

CCLV. When a piece of iron or steel is brought 
within a certain distance of one of the poles of a 
magnet, it is attracted by the latter; and this attrac- 
tion is always strongest at the poles of the magnet. 

Obs. Both magnetic poles together will attract a much greater 
weight than either pole separately. Hence, as the two poles of 
a magnet are generally in opposite parts of its surface, in which 
situation the same piece of iron can not be adapted to them both 
at the same time, two broad pieces of soft iron may be applied to 
the poles of a magnet, and projecting on one side, so that in this 
case, the pieces of iron being rendered magnetic, an other piece of 
iron can be conveniently adapted to their projections, so that both 
poles may act unitedly at the same time. Those pieces of iron, 
palled the Annature, are generally held fast upon the magnet by 
means of a silver or brass box ; and a magnet, thus surrounded 
with plates of iron or steel, is said to be Armed. 

An armed magnet will attract with much greater force and effect 
than an equal one without any armature. The armature, how- 
ever, is not essential ; for artificial magnets have been constructed 
in the shape of a horse-shoe, having their poles in the truncated 
extremities ; in consequence of which both poles ^ill act unitedly 
without any armature. These are hence called Horse-Shoe 
magnets. 

CCLYI. The attraction and repulsion of mag- 
netism is not sensibly affected by the interposition 

€>2\ What is to be understood by the Dipping or Inclination of the 
magnetic needlo I 630. Is the joint attraction of both magnetic poles 
biore powerful than that of either separately 1 631. What is an Arme<$ 
magnet ; and how is it made ? 



HAQHgTlSM. 185 

of bodies of any sort except those which are fer- 
ruginous, or which are attractable by the magnet; 
and the magnetic influence requires some time to 
penetrate through such bodies. 

Exp. 1. Suppose a magnet, placed at an inch distance from a 
piece of iron, requires an ounce of force to remove it ; or which is 
the same thing, suppose that the attraction towards each other is 
equal to one ounce ; it will be found that the same degree of at- 
traction remains constantly unaltered, though a plate of other 
metal, glass, paper, &c. be interposed between the magnet and 
the iron, or though they be inclosed in separate boxes of glass or 
other matter. 

2. Move steel filings placed on a brass plate, in water, &c. by 
holding a magnet under the vessel. 

3. Strew on a sheet of pap r, some iron tilings, and lay a small 
magnet among them ; then shake the table a little, and the filings 
will arrange themselves in the way represented in Fig. 69. But 
if iron filings are shaken through a ?auze sieve upon a paper that 
covers a bar magnet, the filings wijl be arranged in curved lines. 

4. Sprinkle steel dust on a sheet of paper, under which is placed 
a magnet, or two magnets, having their poles opposite to each 
other, and at the distance of about an inch. 

5. A needle under an exhausted receiver, will be attracted at 
the same distance as in the open air. 

6. Place a bulky piece of iron, weighing 40 or 50 pounds, so 
near to a magnetic needle as to draw iFa liitleout of its direction; 
then apply one of the poles of a powerful magnet to the other 
extremity of the iron, and several seconds will elapse before the 
needle can be affected by the magnet ; the interval of time being 
greater or less according to the size of the iron an I the strength 
of the magnet. 

CCLYIL Soft iron is attracted by the magnet 
more forcibly than steel, but it is not capable of 
preserving the magnetic property so long. 

CCLV1II. Heat weakens the magnetic power, 
and a great heat destroys it entirely. The gra- 
dual addition of weight to a magnet, kept in its 
proper situation, increases the magnetic power. 

632. Is the magnetic attraction and repulsion affected by the interpo- 
sition of any substance? 633. How js this proved 7 634. Which is 
attracted more powerfully, iron or steel 1 635. How is the magnetic 
power weakened or destroyed 1 636. In what manner may the magnetic 
power be increased 1 

16* 






186 MAGNETISM. 

Obs. Among natural magnets, the smallest generally possess a 
greater attractive power, in proportion to their size, than those 
which are larger. There have heen natural magnets nor exceed- 
ing twenty or thirty grains, which would lift a piece of iron that 
would weigh forty or fifty times more than themselves. A small 
magnet, worn by Sir Isaac Newton in a ring, weighing about 
three grains, is said to have taken up 746 grains, or near 250 times 
its own weight, and Mr. Cavallo has seen one of six or seven 
grains weight, which was capable of lifting a weight of 300 grains. 
But magnets of two pounds and upwards, seldom lift ten times 
their own weight of iron. 

CCL1X. The north pole of a magnet is more 
powerful in the northern, and the south in the 
southern parts of the world. 

CCLX. When a magnet is divided into two 
parts through the middle, or in any way broken 
in two, each piece becomes a complete magnet, 
possessed of the opposite polarities, the parts 
which were contiguous becoming the opposite 
poles. 

CCLXI. A magnetic needle, freely suspended 
or accurately balanced on a pivot or centre, with 
no iron or other perturbating substance near it, 
will always settle in a certain direction, called 
the magnetic meridian, which is either exactly 
or very nearly north and south. 

The directive power of the magnet is the most wonderful and 
useful of its properties ; and it is this principle of polarity which, 
renders it so useful in navigation ; as by it, navigators are enabled 
to conduct their ves-els through vast oceans in any required di- 
rection, — by it, miners are guided in their subterranean operations ; 
and by it, travellers find their way through deserts otherwise im- 
passable. 

Obs. 1. When a magnet is kept freely suspended, so that it 
may turn north or south, the pilot, by looking at its position, can 
steer his course in any required direction at sea. 

2. An artificial magnet, or magnetic needle, fitted up in a proper 

G'17. When a magnet is divided into two parts through the middle, 
what then? 63 \ Under what circumstances does a magnet place itself 
in the magnetic meridian? 6J9. What are the uses of the magnet's 
directive power? 



MAGNETISM. 187 

box, and furnished with a paper cnrd containing the representation 
of the horizon, is usually called the Mariner's Compass. — See 
Fig. 69 and 70. 

3. Though the north pole of the magnet always points towards 
the northern, and the south towards the southern parts of the 
world, yet their direction is seldom in the exact direction of the 
poles of the Earth, that is, the magnetic and the real meridians 
seldom coincide, and the angle which they make is called the 
Angle of Declination, or Variation of the magnetic needle. 

4. This declination is said to be east or west, according as the 
north pole of the needle is eastward or westward of the- true meri- 
dian of the place, or true north point of the horizon. 

5. At present the declination, or variation of the magnetic 
needle, is about 24|° westward at London ; and the dip, or in- 
clination, about 72° ; but at the North Cape, in Norway, the vari- 
ation is only about 1° towards the west, while, in some parts of 
Davis's straits, its direction is more than 73° towards the west ; 
and at Cape Horn, it is about 22° towards the east of the true 
north point of the horizon. At New- York, the present variation 
is about 4° to the west. 

CCLXII. When a piece of iron is brought 
sufficiently near a magnet, it becomes itself a 
magnet. Bars of iron that have stood long in a 
perpendicular situation, are generally found to 
be magnetical. 

Obs. I. If a long piece of hard iron be made red hot, and then 
suffered to cool in the direction of the magnetical line, it becomes 
magnetical. The electric shock will often render iron magnetical ; 
so also will lightning. 

2. Artificial magnets are made by applying one or more power- 
ful magnets to pieces of hard steel. The power of a magnet is 
not diminished by communicating its properties to other bodies. 

3. Two or more magnets joined together may communicate a 
greater power to a piece of steel than either of them possesses 
singly. 

4. The phenomena of the compass, and the dipping needle, and 
of the magnetism acquired by an iron bar in a vertical position, 
leave no room to doubt that the cause exists within the Earth. 
Dr. Halley supposed that the Earth has within it a large magnetic 
globe, not fixed within to the external parts, having four magnetic 

6.0. What is a Mariners Compass? 6 : l. What is meant by the 
Jtnglt of Declination, or Variation ?■ 6 2 What isthe present variation 
of trie magnetic needle at London— at the North Cc.pe — Davis's Straits — 

at. Cape Horn — and at New- \. ork l 643. In what cases does iron become 
magnetic without the use of a magnet ? 644. How are artificial magnets 
made 1 645. Where does the cause of magnetism exist 7 



188 MAGNETISM. 

poles, two fixed and two moveable, which will account for all the 
phenomena of the compass and dipping needle. This would 
make the variation subject to a constant law, whereas we find 
casual changes which can not be accounted for upon this hypo- 
thesis. This the Doctor supposes may arise from an unequal and 
irregular distribution of the magnetical matter. The irregular 
distribution also of ferruginous matter in the shell may likewise 
cause some irregularities. 

5. Mr. Cavallo's opinion is. that the magnetism of the Earth 
arises from the magnetic substances therein contained, and that 
the magnetic poles may be considered as the centres of the polari- 
ties of all the particular aggregates of the magnetic substances ; 
and as these substances are subject to change, the poles will 
change. Perhaps it may not be easy to conceive how these sub- 
stances can have changed so materially, as to have caused so 
great a variation in the poles, the position of the compass having 
changed from the east towards the west about thirty-three degrees 
in two hundred years. Also the gradual, though not exactly re- 
gular, change ol variation shows that it can not depend upon the 
accidental changes which may take place in the matter of the 
Earth. 

6. Mr. Churchman, of America, says there are two magnetic 
poles of the Earth, one to the north and the other to the south, at 
different distances from the poles of the Earth, and revolving in 
different times ; and, from the combined influence of these two 
poles, he deduces rules for the position of the needle in all places 
of the Earth, and at all times past, present, and to come. The 
north magnetic pole, he says, makes a complete revolution in 426 
years, 77 days, and 9 hours, and the south pole in about 5459 
years. In the beginning of the year 1777, the north magnetic 
pole was 76° 4' north latitude ; and in longitude from Greenwich 
140° east; and the south was in 72° south latitude, 140° east 
from Greenwich. 

CCLXIIT. A magnetic needle is made by fast- 
ening the steel on a piece of board, and drawing 
magnets over it, from the centre outwards. 



Figs. 67 fy 68. 




Ulus. 1. A magnetic needle is 
■■ — represented in Figs. 67 & 68, the 
first of which shows the upper 
side, and the second a side-view 
;of the needle, having a pretty 
large hole in the middle, to whicn 



646. What was the opinion of Mr. Cavallo concerning the cause of 
magnetism 1 647. What is the result of Mr. Churchman's investigations 1 
648. How is a magnetic needle made 1 



ASTR0N03IY. 189 

a conical piece is adapted by means of a brass piece O, into this 
the agate cap, as it is called, is fastened. The apex of the hollow 
cap rests upon the point of a pin F. which is fixed in the cenLre 
of the box, and upon which the needle being properly balanced, 
turns very nimbly. 

lllus. 2. A mariner's compass is represented in Fig". 69 ; the 
box which contains the card, or fly, with the needle, is made of a 
circular form, either of wood, or brass, or copper. It is suspended 
Fig. 69. Fig. 70. 




within a square wooden box B, Fig. 70, by means of two con- 
centric circles, called gambols, so fixed by cross axes, a, a, a, a, 
Fig. 69, to the two boxes, that the inner one, or compass-box, 
shall retain a horizontal position in all motions of a ship, whilst 
the outer or square box is fixed, with respect to the ship. The 
compass-box, is covered with a pane of glass, that the motions of 
the card may not be disturbed by the wind. What is called the 
card is a circular piece of paper, which is fastened upon the needle, 
and moves with it. The outer edge of this card is divided into 
360 equal parts or degrees, and within the circle of these divisions 
it is again divided into 32 equal parts, or arcs, which are called 
the points of the compass, or rhumbs, each of which is often sub- 
divided into quarters. 

3. There seems to be a similarity between magnetism and 
electricity. If two pieces of soft iron wire be led each to a sepa- 
rate thread, and they are hung freely, and if the north end of a 
magnet bar be brought urder them, the wires will repel each 
other, as in electricity. The same result would happen if the 
south pole of the magnet be presented instead of the north. 

ASTRONOMY. 

CCLXIV. Astronomy is the science which 
explains and investigates the nature and motions 
of the Earth, the Sim, Moon. Planets, Comets, 



649. How may the construction of the mariner's compass be illustrated 
by means of a figure or diagram ? 



190 ASTRONOMY. 

and Sta?~s; their periods, eclipses magnitudes, 
distances, and the various other phenomena or 
appearances which these bodies Exhibit ; and the 
laws or principles by which their motions are 
regulated. 

This science may be considered under three principal or general 
divisions, — Plane, Physical, and Descriptive Astronomy. * 

The determination of the magnitudes, distances, and orbits of 
the celestial bodies, is called Plane or Pure Astronomy ; and that 
part of the science, the object of which is to explain and investi- 
gate the causes of their motions and other phenomena, is called 
Physical Astronomy. 

The nature of the luminous objects- that are seen in the heavens, 
and of which the Moon is nearest to us, their number, and tho 
appearances or phenomena occasioned by their movements, to- 
gether with the useful purposes to which the human species has 
applied ihe knowledge of those particulars, constitute that part of 
the science which has been denominated Descriptive Astronomy. 

Obs. l. The student in the day time may observe one of the 
chief of these motions in the rising, ascent, exaltation, declension, 
and setting of the Sun. In the morning, he will see it rise in the 
eastern part of the heavens, ascend in this hemisphere towards 
the South, attain its greatest height at noon, and then descend 
again, till it sets in the West, about as far from the South as it 
rose in the morning. This is the first practical lesson in Astro- 
nomy. 

2. In the night time, he may observe the stars rise in the East, 
ascend towards the South, and decline and set in the West ; and 
this will be the second lesson. 

3. He may, however, observe that one star, or that near the 
North Pole, never seems to move, and that all the others move 
round it, and those within a certain distance never set ; and, in 
short, in this third lesson, which is worthy of being pursued 
through successive evenings, he will become master of the gene- 
ral motions of the heavens. 

4. He will observe, in a fourth lesson, which is also to be pur- 
sued through successive nights, that the Moon changes its place 
with regard to the stars, that it increases in light in proportion to 
its increased distance from the Sun, till it arrives at the opposition, 
or rises as the Sun sets ; and that the light increases on one side, 
and decreases on the other, being always towards the Sun. 

5. He will in like manner observe, that the planets change their 
motions slowly in regard to the fixed stars, and that both Moon 

650. What is Astronomy ! 051. What is Plane Astronomy ? — Physi- 
cal Astronomy 1 — and Descriptive Astronomy I 652. What is the prin- 
cipal and most obvious of the celestial motions? 



ASTRONOMY. 191 

and planets move in the same line, or nearly so, among the fixed 
stars ; and this fifth lesson may be mixed with others through 
successive weeks. 

6. He will be highly gratified by applying any telescope, even 
of very moderate magnifying power, to almost any part of the 
heavens in a clear evening, and also to the Sun, Moon, planets, 
and stars. He may observe the spots upon the disk of the Sun 
and Moon ; the decrease and increase, and change of the shadows 
of the eminences and depressions or pits on the surface of the 
Moon ; the constant change and motion, and the appearance and 
dissolution of the solar spots ; the satellites of Jupiter and Saturn ; 
the luminous ring which surrounds Saturn ; the phases or Moon- 
like appearance of Venus ; many bright spots or luminous spaces 
in the heavens, called Nebula; -and the vast numbers of small 
stars and clusters in many of the constellations, and of which the 
Milky- way and most of the nebulous stars are composed. 

7. By mixing such observations on the heavens and heavenly 
bodies, with the results given in the following paragraphs, of 
observations made during many thousand years, the student will 
soon become expert in this most sublime ot all the sciences. 

CCLXV. The celestial bodies of which we 
have any knowledge, may be classed under two 
general divisions ; — First, with regard to their 
visibility, or the manner in which they appear 
luminous or bright ; and Secondly, with respect 
to their nature, positions, and movements. 

I. The first division of the celestial bodies, is into Luminous 
and Opaq ie. 

1. Luminous bodies are such as give light of themselves ; and 
such are the Sun and Fixed Stars. 

2. Opaque bodies are such as do not emit any light of them- 
selves, though they are capable of reflecting it, when it is cast 
upon them from other bodies ; and they can not, therefore, be seen, 
but by the light from some luminous body shining on them. Of 
this class are the planets and comets, which appear luminous be- 
cause they reflect the light of the Sun and stars. 

Obs. The comets, however, besides the light that they may 
have from the Sun, seem to shine with a light that appears to be 
nothing more than an accension, wmich they receive from the 
Sun, at the time of their near approaches to that luminary, in 

653. What are the two general divisions of the celestial bodies 1 654. 
1 What is the First general division of the celestial bodies ] 655. What-is 
1 the Second general division ? 656. What are the Luminous celestial 
I bodies ? 657. Which are Opaque celestial bodies 7 



192 ASTRONOMY. 

their respective revolutions. The comets are undoubtedly, in 
some degree, self-luminous or luminous per se, whether that 
luminosity arises from an accension, or from some other cause. 

II. The second division of the celestial bodies, is, into primary 
planets, secondary planets, comets, and fixed stars. 

1. The Primary Planets are those celestial bodies that are 
found, by the aid of telescopes, to be large, globular, opaque bodies, 
which revolve about the Sun as their common centre of attraction. 
They are called Planets, or Wandering- Stars, from a Greek 
word, which signifies wandering-; because they change their 
places continually, and seem to wander about amongst the fixed 
stars. 

Obs. The planets, although opaque bodies, appear to us bright 
or luminous, as the fixed stars do, because they arc illuminated 
by the rays of light from the Sun. 

On further examination, according to rational principles, and 
from observation and experience, it becomes evident that the 
Earth which we inhabit, is also a primary planet, moving amongst 
the fixed stars, or which would, to a spectator situated on one of 
the planets, appear to move amongst tne fixed stars precisely in 
the same manner as those planets appear to us to move, and which 
are all found to revolve about the Sun as their common centre, at 
different distances, and in different periodical times. 

2. The Secondary Planets are smaller opaque bodies, which 
revolve about their primaries, and are, at the same time, carried 
with them round the Sun. 

Obs. The secondary planets are also called Satellites, or Moons. 
Our Moon is a secondary planet or satellite, which accompanies 
the Earth, and is carried" with it round the Sun once a year, but 
which revolves round the Earth in a much shorter space of time, 
or about once a month. With the exception of the Moon, .all the 
secondary planets, by which most of the primary planets are 
accompanied, are entirely invisible to the naked eye. 

3. The Comets are those extraordinary celestial objects which 
are commonly accompanied by a luminous train or blaze, called 
the Tail, and which are also found to be large, globular, opaque 
bodies, like the planets ; and, like them, they perform their revo- 
lutions about the Sun, but in very long elliptical orbits, near one 
of the extremities of which the Sun is situated. 

4. The Fixed Stars are those brilliant points or bright luminous 
bodies in the heavens, which appear to shine with a constant 
twinkling or scintillation, and which always appear to us to pre- 
serve the same situation in regard to each other, or which do not 

658. What is said concerning the luminosity of the comets ? 659. 
What is the Second general division of the celestial hodies? 660. What 
are the Primary Planets? 661. Whence is the term planet derived'/ 
662. What is tiie Earth ascertained to the? 661 What are the Secondary 
Planets? 661. By what other names are the secondary planets desig- 
nated ? 665. What are the Comets ? 



THE SOLAR SYSTEM. 193 

sensibly change their relative situations, during a long succession 
of ages. Hence the name Fixed. 

Obs. Our Sun is supposed to resemble these stars in its nature 
and fixed position, and is therefore properly classed with them. 
The Sun, at the same distance from us that the fixed, stars are, 
would probably have the appearance of one of them, or would 
appear, in respect of size and brilliancy, no larger than one of the 
fixed stars. 

CCLXVI. The assemblage or collection of 
those celestial bodies which appear bright and 
luminous in consequence of the light reflected 
from their surfaces, or which shine by the light 
that they receive from the Sun, which is a 
luminous body, constitutes that part of the uni- 
verse, which is usually denominated the Solar 
System. 

CCLXVIL The Solar System consists, 
therefore, of the Sun, which is the only lumi- 
nous body belonging to the system, and is con- 
ceived to be situated in or near the centre ; 

Of eleven Primary Planets, so called because 
they perform their revolutions immediately or 
directly round the centre in their respective 
periodic times ; Mercury ^ , Venus 9 , the 
Earth © , Mars $ , Vesta 6 , Juno £ , Ceres H , 
Pallas £ , Jupiter % , Saturn h , and Uranus or 
Herscnel #; 

Of eighteen Secondary Planets, or satellites, 
the Earth's Moon, Jupiter's four satellites, Saturn's 
seven, and six belonging to Uranus ; 

And of a considerable but indeterminate num- 
ber of comets. 

666. What are the Fixed Stars ; and why so called 1 667. What is 
the hun supposed to resemble 1 668. What kind of celestial bodies con- 
stitute the Solar System ? 669. Of what does the Solar System consist? 
670. What, are the names and characters usually applied to the primary 
planets composing the tolar System 1 671. What is the number of the 
secondary planets, and to what primary planets do they belong 1 
17 









194 ASTRONOMY. 

Obs. Various hypotheses or ideal systems of the universe have 
been invented, for the purpose of accounting for the natural ap- 
pearances and apparent irregularities of the celestial motions. 
The most remarkable of these systems are the Ptolemaic, the 
Tychoriic, and the Copemica.n, or Pythagtm 

1. The Ptolemaic System, so called from Cladius Ptolemceus, 
or Ptolemy, a celebrated Egyptian philosopher, who flourished at 
Alexandria, in the second century of the Christian era, or about 
130 years after the birth of Christ. 

According to this system of Ptolemy, the inventor endeavoured 
to establish the vulgar idea, which is derived from first appear- 
ances, uncontrolled by reason and observation. He supposed 
with the vulgar, that the Earth was perfectly at rest, or fixed and 
immoveable in the centre of the universe, and that all the celestial 
bodies — the Sun, Moon, planets, comets, and fixed stars, performed 
their revolutions about the Earth every day, or in a space of time 
equal to 2 1 hours ; but he farther supposed, that, besides this diur- 
nal motion, t lie Sun, Moon, planets, and comets, had a motion in 
respect of the fixed stars, and were situated with regard to their 
distances from the Earth, in the following order ; the Moon was 
nearest to the Earth, Mercury next, then Venus, the Su i, Mars, 
Jupiter, and Saturn, the other planets not being known at that 
time. The revolutions of these bodies he supposed to be per- 
formed in circles about the Earth placed a little out of the centre, 
with respect to those circle's or orbits. Beyond the sphere of 
Saturn's motions, he conceived the fixed stars to be placed ; and 
here he supposed the existence of various immense orbs, which 
he called the starry firmament, the crystalline orbs, the primum 
mobile, and last of all, the ccelum empyrcum, or heaven of heavens ; 
all of which were imagined to turn round the Earth, once in 24 
hours, besides their having proper and peculiar movements, in 
certain slated or periodical times, agreeably to their various 
changes and appearances. The empyrean heaven, or heaven of 
heavens, however, which was imagined to be situated far beyond 
all the other orbs, and being the residence of the Deity and blessed 
spirits, had no connexion with the material spheres or their mo- 
tions. Every star was supposed to be fixed in a solid transparent 
sphere, like crystal ; and to account for the different motions of 
the Sun, Moon", planets, comets, &c, in respect of the fixed stars, 
he was necessitated to conceive a number of circles called eccen- 
trics and epicycles, which crossed and intersected each other in 
various directions. And if any new celestial motion was dis- 
covered, a new heaven of crystal was formed in order to account 
for the phenomena relative to it ; so that, according to the hu- 
morous observation of Fontenelle, heavens of crystal did not cost 
him any thing, and he multiplied his imaginary crystals without 
end, to answer every purpose. 

(,7J. What are the moot celebrated hypothetical or ideal systems of 
Astronomy 1 



THE SOLAR SYSTEM. 195 

This ridiculous and truly absurd doctrine, although long since 
rejected by enlightened astronomers and mathematicians, was 
supported by many of the ancient philosophers, particularly 
Aristotle, and, indeed, by almost all the subsequent astronomers, 
for nearly J 400 years. 

2. The system received by the Egyptians was as follows; — 
The Earth was supposed to be immoveably fixed in the centre, 
about which revolved, in order, the Moon, Sun. Mars, Jupiter, and 
Saturn, and about the Sun revolved Mercury and Venus. This 
hypothesis will, in some, measure, account for the phases and 
movements of Mercury and Venus, but not for the apparent mo- 
tions and retrogradations of Mar 4 :. Jupiter, Saturn, and Uranus. 

3. The Copemican, or True Solar System, which is now uni- 
versally received amon&rst mathematicians and astronomers, and, 
indeed, admitted by all who possess any share of reason and 
common sense, is not only the true] but also the most ancient 
system in the world, or of which we have any account ; and is, 
in truth, more consistent and even more conformable to natural 
appearances and the circumstances of the celestial motions, than 
any other hypothetical system of Astronomy, that has ever been 
propagated amongst mankind. 

This system was first of all. as far as the moderns have been 
able to ascertain, introduced into Greece and Italy, about 530 years 
before the commencement of the Christian era, by the great Py- 
thagoras and his disciples, from whom it has been called the 
Pyth agorea n System . 

The opinions of Pythagoras, though perhaps derived originally 
from some Eastern nation, were held by some of his pupils ; but 
were either too contradictory, or not sufficiently obvious and clearly 
explained, to be maintained in continual succession, during a se- 
ries of ages, emphatically styled the dark ages — a period of the 
most consummate ignorance and barbarism. The system of 
Pythagoras was, however, adopted and retained, until Plato, 
Aristotle, and the philosophers that came after them, embraced 
the vulgar idea, or what was afterwards called the Ptolemaic 
system ; and their authority imposed it upon mankind, till the 
doctrine which had been taught by Pythagoras more than 2000 
years before, and which had been rejected and in a great measure 
become lo^t or extinct, was revived ab ut the year 1510, by the 
genius of Nicholas Copernicus, an eminent astronomer, born at 
Thorn, in Prussia, A. D. 1473 ; from whom it has been denomi- 
nated the Copernican System. Copernicus having adopted and 
greatly improved the Pythagorean or true system of the universe, 
published it to the world with new demonstrative arguments 
in its favour, in his work entitled, De Revolutionibus Qrbium 
Ccelestlum. first printed at Nuremburg, in 1543, a little previous 
to his death, which took place in the same year, at the age of 
seventy. The truth of the ancient Pythagorean system, by ap- 
plying mathematical reasoning to mechanical experiments, was 



196 ASTRONOMY. 

afterwards completely established by the incomparable Sir Isaac 
Newton. 

4. Although the Copernican System was admitted by most men 
of science then living-, yet some there were who would never 
assent to that theory, and even some men of great learning and 
eminence, such as the celebrated Danish astronomer Longomon- 
tanus, author of the Astronomia Danica, and other works. But 
amongst those who more particularly distinguished themselves 
in opposing the system of Copernicus, was the famous Tycho 
Brahe, a Danish nobleman, who was born at Knudstorp, in 1546, 
and who devoted the wh >le of his life to the study of Astronomy. 
This distinguished character, who has otherwise rendered essen- 
tial services to Astronomy, seems to have admired the simplicity 
and beauty of the Copernican system ; but, supposing this theory 
relative to the Earth's motion to be inconsistent with certain pas- 
sages of the Bible, which seemed to him to contradict the doctrine 
of the Pythagorean system, which Copernicus had lately revived, 
he rejected it and endeavoured to substitute a better ; and as he 
could not entirely adopt the Ptolemaic hypothesis, being convinced 
that the Earth is not the centre about wnich the planets revolve, 
his system is a kind of mean between the Ptolemaic and Coper- 
nican, or seems to partake, in some measure, of both these systems. 
Tycho, entertaining a great respect for the Scriptures, and being 
anxious to reconcile the appearances of nature with certain pas- 
sages contained therein, and taken according to their literal inter- 
pretation, imagined the Earth to be placed immoveably in the 
centre of the universe, as also the centre of the orbits of the Sun 
and Moon, and without any rotation about its axis, thus retaining 
the most ridiculous part of the Ptolemaic hypothesis ; but he sup- 
posed the Sun to be the centre of the orbits of the other planets, 
which therefore revolved with the Sun round the Earth every day, 
or in a space of time nearly equal to 24 hours. The rive primary 
planets then known, he thought moved round the Sun in their 
respective periodical times, Mercury being nearest to the Sun, 
then Venus Mars, Jupiter, and Saturn. 

The new system of Tycho, was published by its inventor 
in opposition to that of Copernicus, about the year 15S6 ; but 
it did not succeed, and was not of long duration, though he 
had many followers, soon after it made its appearance in the 
world. 

5. In regard to the supposition that the whole universe, to its 
farthest visible limits, was carried by the primum mobile about 
the axis of the Earth continually every day, Tycho was abandoned 
by some of his followers, who, seeing the absurdity of a diurnal 
revolution of the celestial bodies round the Earth, and choosing 
rather to save this immense labour to the spheres, ascribed to the 
Earth a diurnal or rotatory motion about its axis ; and supposed 
likewise that the Sun, with all the planets, moved round the Earth 
once a year ; on which account this modified system has been dis- 



THE SOLAR SYSTEM. 197 

tinguished by the name Semi-Tijchonic Si/stem, or its adherents 
called Scmi-Tychmvcs. 

6. An other singular system of Astronomy, distinguished as the 
famous System of Vortices, which at one time made considerable 
noise in the philosophical world, may not improperly be taken 
notice of in this place, particularly as it is the system of the cele- 
brated Descartes ,a French philosopher and mathematician of great 
eminence and celebrity, born at La Haye, in Touraine, 3Iarch 31, 
1596. 

According to the system of Descartes, the Sun is supposed to 
occupy a fixed and central position in the midst of an immense 
whirlpool of subtile or attenuated matter, which extends to the 
utmost bounds o; the system; and the planets being immersed or 
plunged into such parts of this vortex or whirling substance, as 
are equal in density with themselves, are continually drawn along 
with it, and carried round the Sun in their respective orbits by its 
constant circulation. Such of the primary planets as are accom- 
panied by satellites are likewise imagined to be the centres of 
other smaller whirlpools which swim or circulate in the great 
vortex; and the bodies that are situated in the smaller vortices, 
are urged or driven round their primaries, for the same reason and 
in the same manner as those primaries are urged round the Sun ; 
that is, by the constant action or impulsion of the circulating 
fluid. Now as the Sun turns on its axis in the same direction that 
the primary planets move round that luminary, and the primary 
planets also revolve about their axes in the same direction as their 
satellites revolve round them, it was imagined, that if the whole 
planetary region were filled with a fluid matter, of the nature of 
that mentioned above ; the Sun and planets by a rapid motion on 
their axes, would communicate a circular motion to every part of 
this circumambient medium, and by that means draw along the 
bodies that are immersed or swim therein, and give them the same 
circumvolution ! 

At present, it is considered altogether superfluous to adduce 
objections, or to attempt a formal refutation of any of those ima- 
ginary untenable theories of the celestial motions ; as many of the 
objections are very obvious and will readily suggest themselves to 
the minds of those who are accustomed to reflection, or who are 
likely to derive any advantage from the study of Astronomy. 
Men of science have long since demonstrated the truth of what 
is opposed only by the most illiterate and vulgar of the human 
species. • 

CCLXVIII. The Sun is conceived to occupy 
the centre of the solar system, round which all 
the planets move with remarkable order and har- 
mony, at different distances, in different periodi- 
cal times, and with different degrees of velocity. 



198 ASTRONOMY. 

lllus. In the following diagram the solar system is represented. 
S is the Sun in the centre, surrounded by circles, representing in 
succession the orbits of Mercury, Venus, the Earth and Moon, 
Mars, the four Asteroids ; V Vesta, J Juno, C Ceres, P Pallas, 
Jupiter and his four moons, Saturn, its ring, and seven moons, 
and Herschel and its six moons. The student will find it a 
pleasant exercise to draw the system on larger paper, in which 
the distances should be in exact proportion. 
Fig. 71. 




Obs. 1. The truth of this theory relative to the position ol the 
Sun, and the revolutions of the planets, has been demonstrated by 
mathematical reasoning too long and intricate to be sufficiently 
interesting and comprehensive to warrant its insertion in this 
compendium. But there are several obvious circumstances which 
demonstrate that the planets move round the Sun, and which may 
here be alluded to. 

2. Thus Mercury and Venus always appear in the neighbour- 
hood of the Sun, and therefore, if the Sun revolves round the Earth 



G73. How may the Solar System be explained and illustrated 7 674. 
How is the truth of the Copernican theory demonstrated ? 



THE SOLAR SYSTEM. 199 

as a centre, so must those planets ; but if they did, then the mo- 
tion of each would always appear to the inhabitants of the Earth 
nearly equable, and in the same direction ; whereas now they are 
sometimes stationary, or appear to have no proper motion ; some- 
times they move eastward in reference to the fixed stars, and their 
motion is then called direct, progressive, or in consequentia ; 
sometimes they move westward, or have a retrograde motion, and 
are then said to move in antecedentia ; all which appearances are 
necessary when we admit the Sun to be the centre of their orbits 
and of the Earth's, but wholly irreconcileable with any other 
hypothesis. 

Also, when Mercury and Venus appear in conjunction with 
the Sun, they are sometimes hid behind the body of the Sun, 
and sometimes pass between it and the Earth, appearing like a 
dark spot on the Sun's disk ; but if they have latitude, when in 
their superior conjunction, that is, when beyond the Sun, they 
shine with a face perfectly circular, like a "full-moon. But the 
face disappears in the inferior conjunction, that is, when they are 
between us and the Sun, as the Moon does at its change ; whence 
it is evident that their orbits are between the Sun and the orbit of 
the Earth. Mars sometimes appears in opposition to the Sun, 
which proves that its orbit includes that of the Earth ; and that it 
includes the Sun is plain, otherwise Mars would, in its conjunction 
with the Sun, disappear, like Mercury and Venus, which never 
happens. The same may be observed of Jupiter, Saturn, and 
Uranus. . 

3. The motions of the Earth in its orbit are proved, bv the effect 
of its motion on the apparent motions of the several plauetary 
bodies. These, as the Earth happens to be situated, become sta- 
tionarv, retrograde, or direct, and the variations are exactly mea- 
sured by motions referred to the Earth, like the motions of objects 
ashore, when we are moving in a boat. 

CCLXIX. The Earth is of a globular form, 
or nearly such, which may be inferred from 
various considerations. 

1 . The most decisive argument in favour of the rotundity of the 
Earth, is that which is derived from the well-known fact that the 
Earth has been sailed round at different times by different na- 
vigators ; and this, combined with the observations of those 
navigators, is a sufficient proof that the Earth is spherical, or 
nearly so. 

2. A variety of easy but constant observations proves beyond 
the possibility of doubt, that what, at first sight, appears to be a 
vast flat or plain, is, in truth, a convex surface; and upon farther 

675. How is it demonstrated that the planets revolve round the Sim 1 
676. How are the revolutions of the Earth in its orbit proved 1 



200 



ASTRONOMY. 



enquiry, the convexity is found to be extended quite round the 
Earth. Thus, it is constantly observed by all mariners, that as 
they sail from any elevated objects, such as mountains, rocks, 
steeples of churches, &c. they first begin to lose sight of the lower - 
parts of those objects, and then lose" sight of the higher parts, 
gradually from bottom to top, until they entirely disappear. In 
the same manner, when navigators approach a country, they first 
discover the most elevated parts of that country, and the lower 
parts become visible as the land is approached ; or, the highest 
parts of the country are seen from the tops of the mast, and then 
gradually the lower parts become visible, or the same parts become 
visible from the deck of the ship. 

The Sun is observed sooner at rising, and later at setting, by a" 
person at the mast head of a ship, than by one on deck. 

Also, when a ship recedes from the land, a person on shore will 
first lose sight of the hull, then of the masts and lower parts of 
the sails, and lastly of the topsails, gradually from bottom to top ; 
and when a ship approaches the land, a spectator on shore first 
discovers the upper parts of the masts and sails, and then by de- 
grees the lower parts and the hull, in proportion as the vessel 
comes nearer to the shore. In all these cases, the obstruction to 
the sight arises from the interposed water, on account of the 
universa-1 convexity of the surface. 

3. It has been well ascertained that eclipses of the Moon are 
occasioned by the Earth's shadow upon the Moon ; but in all 
eclipses, notwithstanding the various positions of the 'Earth, this 
shadow is always circular; which is an other proof that the Earth 
is a globe. 

4. An other decisive argument is derived from observing the 
altitude of the north polar star, after travelling north or south a 
considerable number of miles ; and generally, in travelling any 
great distance towards the north, the northern stars appear more 
elevated as we approach them, and the southern stars more de- 
pressed as we recede from them towards the north. When tra- 
velling towards the south, the southern stars become elevated and 
the northern depressed. Were the Earth an extended plane, such 
changes in the positions of the fixed stars could never take place. 

Many other reasons, suggested by philosophers, might here be 
enumerated to prove the rotundity of the Earth, had we sufficient 
room and inclination to take up the reader's time with the relation 
of them ; for, a very little attention and a very little observation 
in travelling either by sea or land, must soon convince any reflect- 
ing porson that the Earth is of a globular form. Indeed, in the 
editor's opinion, there is not even one solitary appearance either 
in the whole celestial sphere or throughout the surface of the 
Earth, that seems in the slightest degree to favour the idea of the 
Earth's being an extended plane, or of any other figure than that 
of a globe. 

677. How is it proved that the Earth is of a globular form? 



DEFINITIONS. 201 

Obs. The figure of the Earth is not, however, perfectly spherical 
— both theory and experience have shown that it is very nearly an 
oblate spheroid, somewhat raised or elevated about the equatorial 
parts, and flattened or depressed about the poles, and the differ- 
ence between the equatorial and polar diameters-, according to the 
latest and most accurate measurements, is about 26 miles, its mean 
diameter being about 7,920 miles, and its circumference 24,880. 

CCLXX. A sphere, on the surface of which 
the various regions of the Earth are geographic- 
ally depicted, or on which the fixed stars and 
constellations are represented, according to their 
apparent places in the concave surface of the 
heavens, is usually called an Artificial Globe. 

Obs. Artificial globes are of two kinds, Terrestrial and Celes- 
tial; and are commonly made for the purpose of aiding the under- 
standing, in the easy solution of several astronomical problems, 
and of instructing students in Astronomy and Geography. They 
serve to give a lively representation of their principal objects ; but 
the lines and figures on both globes, are necessarily imaginary. 

I. DEFINITIONS RELATING TO THE TERRESTRIAL GLOBE. 

Fig. 72. 

Obs. I. The Axis of the 
Earth is an imaginary 
line passing through the 
centie north and south, 
about which the diurnal 
revolution is performed. 
It is represented by the 
line between A B, Fig: 
72. 

2. The Poles of the 
Earth are the extremities 
of this axis, A B. 

3. The Equator is the 
circumference of an ima- 
ginary circle, passing 
round the Earth from 

O 35 east to west, perpendi- 
cular to the axis, and at equal distances from the poles ; as the 
line C D^ 

678. What are the real figure, diameter, and circumference of the 
Earth? 679. What is an Artificial Globe? 680. Into how many kinds 
are artificial globes distinguished i 6S1. What is their use ? 6S2. What 
is the Axis of the Earth 1 683. What are the Poles ? 684. What is the 
Equator ? 




202 



ASTRONOMY. 



4. The email circle E F, is called the Arctic circle; the circle 
G H, is called the Antarctic circle. 

6. The circle north of the equator, I K, is called the Tropic of 
Cancer, that south of the equator. L M, the Tropic of Capricorn. 

6. The spaces between the ends north and south of the equator, 
are called Zones; — that space between the tropics is called the 
Torrid Zone; those between the tropics and the polar circles, aro 
called the 'Temperate Zones ; and those to north of the arctic and 
south of the antarctic circles, are called the Frigid Zones. 

7. Latitude is distance north or south of the equator; Longi- 
tude is distance east or west measured upon the equator, from 
any assumed point. 

Astronomical circles, whether great or small, are mathemati- 
cally divided into 360 equal parts called decrees ; — of course, the 
length of a degree depends upon the magnitude of the circle; a 
degree on the surface of the Earth is about 69^ miles. 

II. DEFINITIONS RELATING TO THE CELESTIAL GLOBE. 

Obs. 1. Since the stars, &c, are represented on a convex sur- 
face, whereas their natural appearance is in a concave one, there- 
fore, in using the celestial globe, the student is supposed to be 
situated in the centre of it, and viewing the stars in the concave 
surface. 

Fix. 73. 

2. The line E B, is the 
Tropic of Cancer. The 
line I D, is the Tropic of 
Capricorn ; the Sun never 
goes north of Cancer nor 
south of Capricorn. 

3. The line C F, is the 
jp Equator or Equinoctial 

Line. 

4. The line B I, is the 
Ecliptic, and indicates the 
path that the Sun appears 
annually to pursue in the 
heavens. It is divided in- 
to 12 equal parts, called 
Signs of the Ecliptic. 

5. The points at which 

685. What are the Arctic and Antarctic circles? 686. What are the 
Tropics! 687. What are the Zones; and how are the" d 
with regard to different portions of the Earth's surface? 6 8. What is 
Latitude; and what is Longitude} 689. How is the circumference of 
supposed t - t>e divided I 69 '. Wh • i concerning 

tial globe 1 691. What is the Tropic of Cancer) and 
what the Ttopic of Capricorn ? 692. What is the Equinoctial, or Ce- 
lestial Equator'} * G'J3. What, is the Ecliptic ; and how is it divided? 




DEFINITIONS. 203 

the ecliptic intersects the equinoctial, G H, are called the Equi- 
noctial Points, or Equinoxes. 

6. Those two points of the ecliptic farthest from the equinoctial 
are called Solstices, or Solstitial Points. 

7. That space in the heavens about 16 degrees in width, through 
the middle of which passes the ecliptic, is called the Zodiac. 

8. The Latitude of a heavenly body is its distance from the 
ecliptic ; — Longitude is distance "from the first degree of Aries. 

9. The Sensible Horizon is an imaginary circle, which appears 
to touch the surface of the Earth and separate the visible part of 
the heavens from the invisible. The Rational Horizon is a circle 
parallel to the former, the plane of which passes through the 
centre of the Earth, and divides the heavens into two equal hemi- 
spheres. 

10. The Poles of the Horizon are two points, the one of which, 
over the head of the spectator, is called the Zenith; the other, 
which is under his feet, is called the Nadir. 

11. A circle which passes from north to south through the 
zenith of any place is called a Aleridian, and is said to be the 
meridian of that place. The meridian of any place passing through 
the poles, and falling perpendicularly upon the horizon, intersects 
it in two opposite cardinal points, called North and South. 

12. The Altitude of any heavenly body above the horizon is the 

Eart of a vertical circle intercepted between the body and the 
orizon, or the angle at the centre of the Earth measured by 
that arc. 

13. The Azimuth of a heavenly body, is the arc of the horizon 
intercepted between the meridian and" a vertical circle passing 
through that body ; it is eastern or western as the body is east or 
west of the meridian. 

14. The Amplitude of a heavenly body at its rising or setting, 
is the arc of the horizon intersected between the point where the 
body rises, and the East or West. 

15. The Declination of any heavenly body, is its distance from 
the equinoctial, and is either northern, or southern. 

16. The Risrht Ascension of any heavenly body, is its distance 
from the first of Aries reckoned upon the equinoctial. 

17. A planet's place, considered as seen from the Sun, is called 
its Heliocentric place, and as seen from the Earth, its Geocentric 
place. 

13. Two planets are said to be in Conjunction with each other, 

694. What are the Equinoctial Points; and what are the So'stitial 
Points ? 695. What is the Latitude of a celestial body ? 696. What is 
the Longitude of a celestial body ? 697. What is meant by the Sensib/e 
Horizon, and what by the Rationed or True Hoi'i-z.on ? 6K What are 
the Poles of the Horizon; and what are they called? 699. What is 
meant by Meridian? 700. What is meant by the Altitude, Azimuth, 
and Amplitude, of a celestial body 7 7J1. What is meant by Declina- 
tion ;nn(\ what by Right Ascension? 702. What is to be understood 
by heliocentric place, and what by Geocentric place i 



204 ASTRONOMY. 

when they have the same longitude, or are in the same degree of 
the ecliptic on the same side of the heavens, though their latitude 
may be different. They are said to be in Opposition when their 
longitudes differ half a circle, or they are in opposite sides of the 
heavens. 

19. The celestial sphere is called Right, Oblique, or Parallel, 
as the equator is at right angles, oblique, or parallel to the 
horizon. 

CCLXXI. As the Earth revolves round its 
axis daily from west to east, the heavenly bodies 
appear to a spectator on the Earth to revolve in 
the same time from east to west, and the alter- 
nate succession of day and night is the effect of 
the revolution of the Earth towards and from 
the Sun. 

Obs. 1. For, all the heavenly bodies appearing to move from 
east to west, while the Earth revolves from west to east, the Sun 
will appear, in each revolution, to rise above the horizon in the 
East, and after describing a portion of a circle, to set in the West, 
and will continue below the horizon, till, by the revolution of the 
Earth, it again appears in the East ; and thus day and night are 
alternately produced. 

2. Further, as any meridian will, by the diurnal motion of the 
Earth, revolve from the Sun to the Sun again in 24 hours, and as 
only one half of the Earth can be enlightened at a time, it is evi- 
dent that any particular place will sometimes be turned towards 
the Sun, and sometimes from it, and, being constantly subject to 
these various positions, will be subject to a regular succession of 
light and darkness ; — as long as the place continues in the enlight- 
ened hemisphere, it will be day ; and when, by the diurnal rotation 
of the Earth, the place is carried into the dark hemisphere, it will 
be night. 

CCLXXII. As the Earth revolves round the 
Sun in 365 days, 6 hours, 9 minutes, Hi seconds, 
the Sun appears to revolve round the Earth in 
the same time, but in the contrary direction. 

703. When are two celestial bodies said to be in Conjunction; and when 
in Opposition? 7 H. What is signified by Right, Oblique, or Parallel 
Spheres ? 70"). In what manner is the alternate succession of day and 
night produced 1 706. In what length of time does the Earth perform a 
revolution round the tun 1 



THE ECLIPTIC. 205 

Obs. 1. It is manifest that the circle in which the Sun appears 
to move, is the same as that in which the Earth would appear to 
move, to a spectator in the Sun. Hence, the apparent place of the 
Sun being found, the true place of the Earth in its orbit is known 
to be 1S0 D distant. 

2. The orbit in which the Earth revolves round the Sun is not 
a circle but an ellipse, having the Sun in one of its foci. For, the 
computations of the Sun's place, upon this supposition, allowing 
for the disturbing forces oi the planets, are found to agree with 
observations. 

CCLXXIIT. The annual revolution of the 
Earth, in common with the rest of the planets^ 
round the Sun, is, in popular language, from 
west, by south to east ; or, to speak more philo- 
sophically, it is according to the Order of the 
Signs ; and the same thing may be affirmed 
with respect to the diurnal rotation. 

Obs. 1 . This imaginary zone or belt, called the Zodiac, which 
extends quite round the apparent concave sphere of the heavens, 
is about 16° in breadth ; and along the middle of it is the ecliptic, 
or circle which the Earth would appear to describe annually to a 
spectator in the Sun, and which the Sun appears to describe an- 
nually as seen from the Earth ; and in the plane of which the 
Earth really revolves. Thus, if we suppose the Earth's orbit to 
be in a smooth, thin, solid plane, passing through the centre of the 
Sun, extending out as far as the starry heavens, and terminating 
in a circle which passes through the middle of the zodiac ; in this 
imaginary plane, the Earth moves in its revolution round the 
Sun ; it is therefore called the plane of the Earth's orbit, and the 
great circle in which this plane intersects the zodiacal spare is the 
ecliptic. And so immensely great is the distance of the fixed 
stars from the solar system, "that in whatever part of its orbit the 
Earth may at any time be situated, the stars constantly appear to 
retain the same order, relative situation, and magnitude. 

2. The ecliptic is supposed to be divided into 12 equal parts, 
which are called Signs. Each sign, therefore, contains 30 de- 
grees. The division of the signs commences at the vernal equi- 
noctial point, and they are numbered in the direction of the Sun's 
apparent motion in the ecliptic, or of the real motion of the Earth. 
The signs of the ecliptic are sometimes designated by names or 
characters, instead of numbers. 

707. In what direction is the annual and diurnal motion of the Earth ? 
708. What is the position of the Ecliptic and Zodiac 1 709. What ore 
the twelve Signs of the Ecliptic 1 

13 






a. 


Names. 


Charac. 


#. 


Na7iies. Charac. 


4. 


Leo, . 


■ si 


8. 


Sagittarius, J 


5. 


Virgo, 


• m 


9. 


Caprieornus, VJ 


6. 


Libra, 


-£V 


10. 


Aquarius, ££ 


7. 


Scorpio, 


. m 


11. 


Pisces, . ^ 



206 ASTRONOMY. 

The names of the twelve signs, with their corresponding num- 
bers, and the characters, by which they are usually represented, 

IS follows ; 

& Names." Charac. 

0. Aries, . °r° 

1. Taurus, . g 

2. Gemini, . XI I 

3. Cancer, . o | 

3. The names and characters or symbols of the signs of the 
zodiac* are the same with those of the signs of the ecliptic ; but 
they do not noio exactly correspond with the latter ; as will be 
illustrated in the sequel. 

4. The space or portion of the starry concave, called the zodiac, 
which contains the twelve constellations or signs, includes the 
orbits of all the planets, or rather their apparent paths as traced 
amongst the fixed stars, excepting, however, Juno, Ceres, and 
Pallas, which have been discovered since the year 1800. 

5. These twelve constellations, and of course, the twelve signs 
of the ecliptic, are so situated, that the Earth in its annual revo- 
lution, passes directly between them and the Sun ; and the latter, 
as it appears to move round annually in the ecliptic, seems to enter 
these clusters of stars, as well as the signs of the ecliptic. For, 
the Sun, besides partaking with the stars in the apparent diurnal 
motion, appears to have a motion eastward amongst the fixed 
stars, along the line of the ecliptic ; the time between the Sun's 
passing the meridian on any day, and its passing the same on the 
next, being found to be greater than a sidereal day, or duration of 
a complete rotation of the Earth on its axis, by a about 3 minutes 
and 56 seconds. 

6. Hence, a straight line, supposed to be drawn from the Earth, 
and passing through the Sun, will always reach one of these con- 
stellations, or sisrns ; and the Sun is sain to be in that constellation 
or sign, at which the imaginary straight line would seem to ter- 
minate, or towards which it is at any time directed. 

170. What are the names and characters of the signs 1 711. What is 
the difference between the signs of the Zodiac and the signs of the 
Ecliptic 7 712. What is meant when the tSun is said to be in a particu- 
lar sign or constellation ? 



* The stars in the zodiac have been classed into twelve groups or 
clusters, and called Sig?is, and from these the expression signs of the 
cdiptir has beer, derived. The constellations or signs of the zodiac, 
therefore, are groups or clusters of stars ; and figures representing these 
■ding to tliC imaginary outlines or figures of the animals 
which the dust «rs of stars were anciently supposed to resemble, are 
usually drawn upon the surface of a hollow sphere, called the celestial 
in that portion of its spherical surface, which corresponds to the 
portion ofthe concave sphere of the heavens, in which the stars belonginr 
to each sign arc respectively situated. 



THE ECLIPTIC. 207 

Thus, since the plane of the Earth's orbit extends or passes 
through the centre of the Sun, it is evident, that whilst a spectator 
in the Sun would see the Earth in the direction of a certain point 
of the ecliptic, an observer situated on the Earth will see the Sun 
in the direction of the opposite point of the ecliptic. 

Fisr. 74. 



NPE 




SPE 

For instance, in Fig. 74, let S represent the Sun in the centre 
of the starry concave in which the external circles, lines, points, 
&c. are supposed to be situated ; the circle L G M. the orbit in 
which the Earth moves, from west, by south, to east, or in the 
order of the letters, L G M, so as to perform the entire revolution 
in one year ; the orbit, for the purpose of rendering it apparent, 
being described vastly larger in proportion to the other circles, 
than" is denoted by the true proportions. The circle A B D P 



713. How is this illustrated ? 



208 ASTRONOMY. 

represents the ecliptic, or imaginary circle, formed by the exten- 
sion of the plane of the Earth's orbit to the starry concave ; and 
the two circles JKXZ and V W O R U represent the broad belt 
or circle in the concave sphere of the heavens, called the zodiac, 
occupied by the twelve constellations, extending on each side of 
the ecliptic quite round the celestial sphere, and conceived to be 
here viewed obliquely,* as well as the equinoctial and orbit of the 
Earth, which is represented in different parts of its orbit by the 
letters Eens, E e Deing the equator, rcsthe axis, n the north pole, 
and s the south pole. Also, the line N P E-S P E, perpendicular 
to the plane of the ecliptic, is the axis of the ecliptic, N P E its 
north pole, and S P E its south pole ; and, in like manner, np e- 
s p e is the axis of the equinoctial, or celestial equator, being in 
the direction of the north pole np e, near which is situated the 
north polar star. 

Now, a spectator at S would perceive the Earth at G, as if it 
coincided with the beginning of the sign Taurus ; when the Earth 
is at M or L, the same spectator would perceive it to coincide 
with the sign Cancer or Capricorn ; and so on. But an observer 
on the Earth, when the Earth is at G, will see the Sun as if it 
were in the sign Scorpio ; when the Earth is at M, the Sun will 
appear in the beginning of Capricorn; and when the Earth is at 
L, the Sun will appear in the sign Cancer, &c. And whilst the 
Earth is moving from L through G to M, it is manifest, that, to a 
spectator on the Earth, the Sun must appear to move from C 
through F to t. In like manner, while the Earth is passing from 
M to L, the Sun will appear to pass from t through A to C. 

7. Hence, it is evident, from the inspection of the preceding 
diagram, that whether we suppose the Sun to revolve round the 
Earth, or the latter round the former, the apparent annual motion 
of the Sun along the ecliptic must be exactly the same ; and it is 
very obvious, that the circle in which the Sun appears to move is 
the same as that in which the Earth would appear to move, to a 
spectator in the Sun. And hence the apparent place of the Sun 
at any given time, being determined, the true place of the Earth 
in itsorbit is readily known, being 180 degrees distant. 

And, the student will do well to remember, that the observations 
founded upon the idea or supposition that the Sun and stars revolve 
round the Earth, are founded upon appearances only, and not upon 
facts. He will therefore not forget, while he is imagining that the 

714. Explain the diagram. 



* In this representation the eye of the spectator is supposed to be 
situated at a vast distance, and viewing the apparent concave sphere of 
the heavens with the imaginary lines, points, circles, <fec. contained 
therein ; and in order to simplify the explanations to be given hereafter, 
we may suppose the diagram to be placed perpendicularly, in such a 
manner that the axis npe-spe may be directed towards the north pole, 
or north polar star, so as to coincide with the axis of the Earth, or be 
parallel to the celestial axis. 



THE ECLIPTIC. 209 

heavens move round the Earth, that, in fact, they are stationary 
as it respects the Earth, and it is the Earth that moves round the 
Sun. 

8. The days on which the Sun appears to enter the signs of the 
ecliptic are exhibited in the following table, together with the 
names of the signs. 



g ( Spring Signs. 

o Aries, 20th of March. 

go I Taurus, 19th of April, 

g I Gemini, 20th of May. 

w ) Summer Signs. 

■5 Cancer, 21st of June. 

g Leo, 22d of July. 

£ t Virgo, 23d of August. 



n C Autumnal Signs. 

% Libra, 23d of September. 
$ I Scorpio, 23d of October. 
& J Sagittarius, 22d November. 
g \ Winter Signs. 

= I Capricornus,22d December, 
b I Aquarius. 20th of January, 
jg t Pisces, 19th of February. 



The Sun does not, however, enter these signs, exactly on the 
same days in the year respectively. 

9. The times at which the Sun enters the signs, as above given, 
must be understood of the signs of the ecliptic, which are reckoned 
from the vernal equinoctial point; and whenever the signs are 
referred to, it must always be understood of the signs of the eclip- 
tic, except when the contrary is mentioned. For, in consequence 
of the retrograde motion of the equinoctial points, which motion 
is to be explained in the sequel, the constellations of the zodiac 
are now more than a sign in advance of the signs of the ecliptic ; 
or, in other words, the signs of the ecliptic, which are always 
reckoned from the vernal equinox, have receded nearly 34 c from 
the signs or constellations of the zodiac, with which they are sup- 
posed to have corresponded about 2400 years ago. Thus, in Fig. 
74, A is the vernal equinoctial point, or that point of the equinoc- 
tial in which the Sun appears, on or about the 20th of March ; 
and A B, B c, c C, &c. are the signs of the ecliptic ; — the con- 
stellations bearing the same names are at present situated with 
respect to the above-mentioned point, as represented or contained 
between the dotted lines a b, df< and df, g h, &c. ; that is, the 
constellation called Aries is contained between the lines a b and 
df; the lines (//and gh are representative of the position of the 
constellation Taurus ; and so of the rest. In a like period of 2400 
years, the equinoctial point A, or intersection of the ecliptic and 
equinoctial will be in r, or about 2^ signs from the beginning of 
the constellation Aries. 

Thus it appears that the constellation Aries is at present 3 or 4 
degrees within the sign Taurus, and in 24 centuries it will be 7 
or S degrees within the sign Gemini; — or, the first point of the 
constellation Aries, which formerly coincided with the vernal 
equinoctial point. A. is now nearly 34° advanced with respect to 

715. On what days does the Sun appear to enter the signs of the eclip- 
tic 1 716. What is said of this retrograde motion of the equinoctial 
points'? 717. How may this be illustrated by the diagram, Fig. 74 ] 
13* 



210 ASTRONOMY. 

that point, in eonsequenoe of its retrograde motion, or motion 
contrary to the order of the signs. However, the first point or 
beginning of the sign Aries still continues to be reckoned from 
the equinoctial point ; and it is from the names of the constella- 
tions that the signs of the ecliptic have been denominated. The 
signs, therefore, must now be distinguished from the constella- 
tions, the signs being merely ideal or imaginary, and serving only 
to designate the " Sun's course in the ecliptic," celestial longitude, 
&c. while the constellations of the zodiac continue to signify groups 
or clusters of stars, designated by particular names. For the sake 
of distinction, some writers have proposed to denominate the 
twelve portions of the ecliptic, Anastrous Signs, that is, si^ns 
without stars, and the constellations themselvos have been called 
Starry Signs. 

CCLXXIV. Theaxis of the Earth in every part 
of the Earth's revolution about the Sun, makes 
with the plane of its orb, that is, of the ecliptic, 
an angle of about 66° 32' ; consequently the 
planes of the equator and ecliptic, make with 
each other an angle of 23° 28' nearly, being the 
complement of 66° 32' to 90°. 

Obs. 1. The obliquity of the ecliptic is not permanent, but is 
continually diminishing by the ecliptic's approaching nearer to a 
parallelism with the equinoctial, at the rate of about half a second 
in a year, or from 50" to 55" in 100 years. — The inclination on 
January 1, 1815, was 23° 27' 46" nearly. The diminution of the 
obliquity of the ecliptic to the equinoctial, is owing to the action 
of the planets upon the Earth, especially the planets Venus and 
Jupiter. The whole diminution, it is said, can never exceed 1°, 
when it will again increase. 

2. According to the calculations of Lagrange, the obliquity 
of the ecliptic has diminished during 2000 years, and will 
diminish during 2000 more; and Schubert has determined its 
limits at 20° 34', and 27° 48'. Its variation in a century is 50" at 

f>resent. The change of obliquity will never exceed a certain 
imit, as is shown by Physical Astronomy ; which "limit, according 
to Laplace, is 2° 42'. 

3. The diminutionof the obliquityof the ecliptic is a consequence 
of the approach of the Earth's axis towards a perpendicular direc- 
tion to the plane of the ecliptic; but the Earth's axis has, besides 
the progressive motion, a tremulous one, by which its inclination 

718. What is the effect of this retrograde motion of the equinoctial 
points 1 719. What is the quantity of the angle formed by the inclina- 
tion of the ecliptic to the equinoctial 7 720. What is said concerning 
the permanency of the obliquity of the ecliptic ? 



TERRESTRIAL PHENOMENA. 211 

to the plane of the ecliptic varies backwards and forwards some 
seconds ; the period of these variations is nine years. The tre- 
mulous motion is termed the Nidation of the Earth's axis. Both 
these motions of the terrestrial axis are occasioned by the action 
of the Sun, Moon, and planets, on the Earth. 

CCLXXV. The axis of the heavens is per- 
pendicular to the planes of all the circles which 
the celestial bodies seem to describe in their 
apparent diurnal motions. For all the celestial 
bodies, from the rotation of the Earth on its 
axis, appear to move from east to west in circles 
perpendicular to the axis. 

Obs. 1. Hence it is evident that the planes of all the circles of 
apparent daily motion are parallel to the equator and equinoctial, 
and that the celestial axis passes through the centres of those 
circles. 

2. When the Sun, or any other celestial body is in the equinoc- 
tial, it rises in the east and sets in the west. For it then rises 
and sets in the points in which the equinoctial intersects the 
horizon ; that is, because the equinoctial is at right angles to the 
meridian, which passes through the north and south points, in 
the points of east and west. 

In north latitude, those celestial bodies which have north 
declination, rise between the east and north ; and those which 
have south declination, rise between the east and south. 

CCLXXVI. Those inhabitants who live at 
the equator are in a right sphere ; and, conse- 
quently, their days and nights are always equal. 

An inhabitant at either of the poles of the Earth would be in a 
parallel sphere ; he would see all the celestial bodies apparently- 
revolving round him in circles parallel to the horizon, and his day 
and night would continue each a half a year. 

But those who live on any part of the surface of the Earth, 
between the equator and either pole, are in an oblique sphere, and 
have all the circles of daily motion oblique to their horizon. 

721. From what cause does the diminution of the obliquity of the 
ecliptic arise? 722. What is the Nutation of the Earth's axis; and 
by what is it occasioned ? 723. What is observed concerning the per- 
pendicularity of the Earth's axis to the planes of all the circles of appa- 
rent diurnal motion ? 724. At what time does the Sun rise in the east 
and set in the west 1 72o. In what direction do those celestial bodies rise 
which have northern declination ? 726. What is said concerning the 
celestial phenomena in a right sphere 1 — in a parallel sphere 1 — and in 
an oblique sphere 1 



212 ASTRONOMY. 

CCLXXVII. When the Sun, in its apparent 
annual course, is in the points in which the 
ecliptic intersects the equinoctial, the day and 
night will be of the same length at all places on 
the surface of the Earth ; but when the Sun is 
in any other part of the ecliptic, the days will 
be longer, as the Sun's declination towards the 
elevated pole increases, and shorter as its decli- 
nation towards the depressed pole increases. 

Obs. All those celestial bodies which are at any time on the 
banie side of the equinoctial with the spectator, continue longer 
above the horizon than below it, and vice versa. 

CCLXXVIII. At different places, the hour of 
the day differs in proportion to the difference of 
longitude ; 15 degrees of longitude making the 
difference of one hour in time, 15' one minute 
of time, 15" one second of time ; and a celestial 
appearance is seen at any given place sooner 
than at places which are situated to the west of 
it, and later at places which are situated to the 
east of it, 

Obs. The Sun, in its apparent diurnal motion, which is fro in 
east to west, must arrive at the meridian of any given place, as 
New-York, sooner than it will arrive at the meridian of any place 
which is situated to the west of New- York, and later than at the 
meridian of any place to the east of New-York ; that is, since it 
is noon at any place when the Sun is in its meridian, it will he 
noon sooner at New-York than at places west, and later than at 
places east of New- York. 

CCLXXIX. The difference of longitude at 
two places, may be found by observing, at the 
same time from both places, some simultaneous 
appearance in the heavens. 

. When are the d tva and nights equal ; and when unequal? 723. 
How docs the hour of tne day differ at different places 1 729. What is 
.f that difference I 730. In wliat manner may the difference 
oflongitude at two places be ascertained 1 



TERRESTRIAL PHENOMENA. 213 

Obs. If the eclipse of Jupiter's innermost satellite, on the very 
instant of its immersion into the shadow of Jupiter, be observed 
by two persons at different places, it will be seen by both at the 
same instant. But if this instant, with reference to the day. be 
half an hour, for example, sooner at one place than at the other, 
because the places differ half an hour in their reckoning of time, 
their difference of longitude must be 7° 30' ; because the whole 
360° are equal to 24 hours, and consequently every 15° are equal 
to an hour. 

CCLXXX. The inhabitants of the Earth have 
different names assigned to them by geographers, 
according to the several meridians and parallels 
of latitude they live under, and are called Pe- 
rioeci, Antceci, and Antipodes. 

1. Those who live on opposite sides of the 
Earth, bat in the same parallel of latitude, have 
opposite hours of the day, but the same seasons. 

Obs. Being both on the same side of the equator and at the 
same distance from it, when the Sun's declination makes it sum- 
mer or winter in one of the places, it will be the same at the other ; 
but because they are distant from each other 130° of longitude, 
when it is noon at one place, it is midnight at the other ; these 
are called Periosci. 

2. Those who live in opposite parallels of lati- 
tude, but under the same meridian, have opposite 
seasons of the year, but the same hour of the 
day. 

Obs. When the Sun has declination towards the north pole, it 
will be summer to those who live in the northern parallel of lati- 
tude, and winter to those who live in the southern parallel of lati- 
tude. But having the same longitude, their hours of the day will 
be the same ; these are called Antoeci. 

3. Those who live in opposite parallels of la- 
titude and opposite semicircles of the meridian, 
have opposite seasons of the year, and opposite 
hours of the day. 

731. How is the manner of ascertaining the difference of longitude at 
two places explained 1 732. How are the inhabitants of the Earth deno- 
minated according to their several situations on the surface? 



214 ASTRONOMY. 

Obs. 1. Because they are in opposite latitudes, they will have 
opposite seasons; andoecanse tney are in opposite semicircles 
of the meridian, they will have noon when it is midnight at the 
other place ; these are called Ant podes, 

2. These and many other propositions will be more readily and 
clearly understood by means of Artificial Globes and the problems 
on the Globes, &c. Sec Ryan's New American Grammar of Astro- 
nomy ; Trecby's Elements of Astronomy ; and other works. 

CCLXXXI. The axis of the Earth, in its cir- 
cuit round the Sun, being inclined to the plane 
or level of its orbit, this inclination occasions the 
succession of the four seasons. 

Obs. 1 . It has already been shown that the alternate succession 
of day and night is produced merely by the simple uniform rota- 
tion of the Earth upon its axis, as must be evident upon a mo- 
ment's reflection ; but the different lengths of days and nights, 
in every part of the world, in the course of the year, and the phe- 
nomena of the different seasons, or those delightful changes which 
we constantly experience, of heat and cold, Summer and Winter 
Spring and Autumn, are occasioned by the annual revolution of 
the Earth about the Sun, in the plane of the ecliptic. 

2. As the Earth's axis makes an angle of 66° 32' with its orbit, 
that is, with the ecliptic, and always preserves its parallelism, it 
is directed towards trie same point, at an infinite distance in the 
heavens ; hence, during one half of the year, the north pole is con- 
tinually illuminated by the San, and the south pole is all that 
time in darkness ; and daring the other half of the year, the south 
pole is constantly in the light, and the north pole is in darkness ; 
and other parts in a proportional degree partake of this vicissi- 
tude, and create the variety of the seasons. 

The difference in the degrees of heat, is owing chiefly to the 
different "heights to which the Sun rises above the horizon and the 
different length of the day?. When the Sun rises highest in sum- 
mer, its rays fall less obliquely, and consequently, more of them 
fall on any given portion of the Earth's surface, than in winter, 
when the rays fail obliquely ; and when the days are long, and 
the nights snort, the Earth and air are more heated in the day 
than they are cooled in the night, and the reverse when the days 
are short and the nights long. 

733. What ore the Periaci, Ant&ci, rind Antipodes! How are the 

relating Co them illustrates ] 7-4. What c 

c seasons, and the difference in the lengths of the days 

an d | EIow much is the axis of the Earth inclined to the 

plane of its orbit 1 736. What is the cause of the difl'crence in the degree* 

of heat 1 



VARIATIONS OF THE SEASONS. 215 

Ittus. In Fig. 75*, which we may consider as enlarged from 
L G M r of Fig. 74, let the inner circle, or rather oval, represent 
the orbit of the Earth, S the Sun in the lower focns of the orbit, 
the four globes. N £ S W, the Earth in different parts of its ellip- 
tic orbit, receiving from its changing position the varying seasons, 
and the succession and different lengths of day and night ; N S 
the Earth's axis, N its north pole, S its south pole. &c. — The 
spectator is here supposed to be beyond or without the orbit of 
the Earth, at a considerable distance, and to look upon it ob- 
liquely. 

As the Earth revolves round the Sun, according to the order of 
the letters, A B C D, &c, its axis N S keeps the same obliquity, or 
always preserves its parallelism, it being, in any one part of the 
orbit, in a position always very nearly parallel to that m which it 
was, in any other part of its orbit, and its northern extremity 
always pointing nearly towards a fixed star situate immensely 
beyond the bounds of the solar system, and hence denominated the 
Aorth Polar Star. 

We shall now suppose the Earth to set off from its position at 
the time of the vernal equinox, in the Spring, and to revolve an- 
nually round the Sun S from the point A. or that part of the orbit 
in which a spectator in the Sun would see the Earth coincide with 
the beginning of the sisrn Libra, of the ecliptic, and the inhabit- 
ants of the Earth see the Sun in the direction of Aries, or when 
the Sun, as seen from the Earth, appears to enter Aries. In this 
position of the Earth, about the 20th of Ma'-ch. the axis, though 
inclined to the plane of the orbit, is not inclined towards the Sun, 
the rays of which for a short time illuminate both hemispheres 
alike, from one pole to the other, causing an equal length oi 
and night in all parts of the world. -At this time, which is called 
the Vernal Equinox, the rays of the Sun fall perpendicularly on 
the terrestrial equator, and more or less obliquely on places more 
or less remote from the equator. 

As the Earth proceeds in its orbit, according to the order of the 
signs, and passes from Libra, along the signs Scorpio and Sagitta- 
rius, to Capricornus, the Sun is seen to advance from the west to- 
wards the east, along the signs. Aries, Taurus, and Gemini, to 
Cancer; until in about three months, that is to say, on the 21st of 
June, the Earth will arrive at the beginning of Ccpricorn. and the 
Sun, as seen from the Earth, will appear at the beginning of 
Cancer. Here, the north pole, in consequence of the obliquity of 
the axis, becomes eventually so inclined towards the Sun, that a 
considerable space round that pole becomes illuminated by the 

737. How are the variations of the seasons illustrated by means of a 
diagram? 73% Explain the several parts of the diagram 1 "739. To what 
part of the heavens is the north pole always directed V 7-iO. When is 
the Earth at the vernal equinoctial point? 

• For Fig. 75, see Frontispiece 



216 ASTRONOMY. 

solar light, and an equal space round the south pole is in the 
shade, in consequence of being turned from the Sun. At this time, 
which is our Summer Solstice, the solar rays fall obliquely on the 
equator, and perpendicularly on a parallel of latitude about 23° 28' 
north of the equator, that is, on the Tropic of Cancer. The space 
then illuminated round the north pole, and that in obscurity about 
the south pole, extend also to the distance of about 23° 23' from 
each pole. 

After this, the days begin to shorten in the northern parts of 
the world, and the nights lengthen, in proportion as the Earth 
continues its course along the signs, Capricornus, Aquarius, and 
Pisces, at the same time that the Sun is seen to advance along 
the signs, Cancer, Leo, and Virgo, the Earth, in its annual revo- 
lution, advancing towards the first point of Aries, at which it ar- 
rives on the 23d of September, when the Sun, as seen from the 
Earth, appears at the beginning of Libra; and at this time, which 
is the Autumnal Equinox, the same conditions occur, as when the 
Earth is in the opposite part of its orbit, at the vernal equinox. 

If we now follow the Earth through the other half of its orbit, 
we shall find, that exactly the same effects take place in the 
southern hemisphere, while the Earth is percurring that half of 
its orbit, as what have been just remarked with respect to the 
northern hemisphere, while the Earth was passing from A through 
B to C. When the Earth arrives at D, about the 22d of Decem- 
ber, the time of our Winter Solstice, the north pole is involved in 
obscurity, and a space round the south pole becomes enlightened 
to the distance of about 23° 23' every way from that pole ; and 
then the solar rays fall perpendicularly on the Tropic of Capricorn, 
the parallel of latitude 23° 28' south of the equator. 

2. Thus we see, that from March to September, or while the 
Earth is moving from Libra through Capricorn to Aries, the north 
pole is inclined towards the Sun, so that the rays of light fall most 
directly on the northern hemisphere, producing summer in the 
northern parts of the world ; the north pole remaining all that 
time in the illuminated hemisphere, will therefore have six months 
of continual day ; — at the same time that the south pole is turned 
from the Sun, which then shines very obliquely on all those parts 
of the southern hemisphere which are enlightened by the Sun, 
thus producing winter in that hemisphere. But during the oilier 
half of the year, from September to March, while the Earth is 
moving from Aries through Cancer to Libra, the north pole is 
turned from the Sun, and therefore in darkness, while the south 
pole is presented more directly to that luminary; and the inhabit- 
ants of the southern hemisphere are cheered with summer, while 

711. When does the summer solstice take place? 742. When is the 
north pole inclined towards the Sun ? What is the situation of the south 
pole -w t!ipo!(;is inclined towards the Sun? 743. When is 

the winter solstice 1 7 14. How does the tun shine upon the Earth at the 
time of the winter solstice 1 



VARIATIONS OF THE SEASONS* 

those of the northern hemisphere are chilled with winter. Hence 
it is easy to perceive, that the inhabitants of the southern hemi- 
sphere have the same vicissitudes with those of the northern 
hemisphere, though not at the same time, it being winter in one 
hemisphere when it is summer in the other. In this way. the two 
hemispheres always have opposite seasons at the same time. 

Thus, in like manner, we see that the inclination of the Earth's 
axis to the plane of the ecliptic, also causes a variation in the days 
and nights in different parts of the Earth's surface, and in different 
seasons of the year ; that at the equator the days and nights are 
equal, as they always are, and the Sun rises and sets at six c' 
throughout the year ; that, about the 20th of March, and the 
of September, which are the periods of the equinoxes, the days are 
equal to the nights in all parts of the world; every part of the 
Earth's surface coming into the light at six in the morning 
going into the dark at six in the evening ; but, that, at other time 
the days and nights vary m length according to the latitude of the 
place and the season of the year. 

Hence also we see why, from March to ScjAcmber, the northern 
hemisphere being more than half enlightened, the days are more 
than 12 hours long; and at the same time, the southern hemi- 
sphere has less than 12 hours of Jigbt; and likewise why. from 
September to March, it being then less than half enlightened, the 
days are less than 12 hours, while the southern hemisphe;\ 
the long days of summer. 

Thus it appears, that, at the equator, the days are always 12 
hours long; from the equator to latitude 60° 32'*. the longesi 
vary from 12 to 24 hours, increasing in length with the lath 
and from the polar circles to the poles, they vary from 24 hou*-s to 
six months, there being alternately at each poll six months light 
and six months darkness. 

CCLXXXII. The orbit in which the Earth 
revolves about the Sun, is not a circle but an 
ellipse or oval. 

Obs. 1. The Sun's apparent diameter being on the 31st of 
December 32' 35.6", and on the 1st of July 31' 31", or about one- 
thirtieth greater, and consequently the Sun so much nearer 
apparently increasing and decreasmo- gradually, it is inferred that 
the distance of the Sun from the Earth is variable ; for there is 
no reason to suppose that this change in the apparent diameter 
of the Sun, is caused by a change in its real diameter, and hence 
it has been concluded that the orbit in which the Farth rev- 
round the Sun, is not a circle, bat an ellipse or oval, having the 
Sun in one of the foci. 



7- 5. What is the figure of the Earth's orbit ? 746. How is the elliptic 
city of t.ie Earth's orbit proved ] 

19 



21S ASTRONOMY. 

Newton found the mean apparent diameter of the Sun, to be 
about 32' 12", but agreeably to the preceding statement, it is 32' 

2. [n January, therefore, the Earth is in its perihelion, and in 
July in it* and having a smaller circle to traverse in 
its perihelion -half than its aphelion-half, it is nearly eight days 

r in performing the aphelion-half of its orb-it than the peri- 
helion-half. 

3. The motion cf the aphelion point is 1° 43' 10" in 100 years i 
so that it will pass round the ecliptic in 20.931 years. 

4. The velocity of the Earth, like that of all the other planets > 
varies in different parts of its orbit ; it being greater in the peri- 
helion, about the 1st of January, and slowest in the aphelion about 
July 1st. The daily motion in the perihelion is 62' 12", and in 
the aphelion 59' 12", being about one-twentieth less ; the mean 
daily motion is 59' 8.3". 

CCLXXXHL Twilight is occasioned by the 
atmosphere above the horizon reflecting rays of 
the Sun, when the Sun itself is below the horizon. 

Obs. L When the Sun is at any point below the horizon, it can 
not be directly seen by a spectator. But, because rays from the 
Sun can pass to the part of the atmosphere above the head of the 
spectator, this part of the atmosphere will be illuminated before the 
Sun rises, or after it sets, and will become visible by reflection to 
the spectator ; that is, twilight will be produced. 

2. It is entirely owing to the reflection of the atmosphere that 
the heavens appear bright in the day time. For, without it, only 
that part would be luminous in which the Sun is placed • and if 
we could live without air, and should turn our backs to the Sun, 
the whole heavens would appear as dark as in the night In this 
case also we should have no twilight, but a sudden transition from 
the brightest sunshine to dark night immediately after the setting 
of tlm Sun. 

3. The twilight is longest in a parallel sphere, and shortest inr 
a right sphere ; and in an oblique sphere, trie nearer the sphere 
approaches to a parallel, the longer is the twilight, because twi- 
light lasts till the Sun is eighteen degrees perpendicularly below 
the horizon. 

CCLXXX1V. The atmosphere also refracts 
the Sun's rays in such a manner, as to bring that 
luminary into sight, every clear day, before it 
rises in the horizon, and to keep it in view for 

717. What is the cause of twilifcht 1 7.3. Haw is the phenomena 
explained? 749. What would he the consequence if the atmosphere 
onihilated ) 730. Where is the twilight longest? 



VARIATIONS OF THE SEASONS. 219 

some minutes, after it is really set below the 
horizon. The effect of this refraction in a right 
sphere is about two minutes of time, or 33' of 
space, being rather more than the diameter of 
the Sun or Moon. 

Qbs. 1. From the same cause, all the hen venly bodies appear 
higher than they really are, so that to bring the apparent altitudes 
to the true ones, the quantity of refraction must be subtracted. 
The higher they rise the less are the rays refrac'.ed, and when the 
heavenly bodies are in the zenith, they sutler no refraction. 

2. When the evening twilight ends, or the morning twilight 
begins, a ray of light from the Sun. reflected from the highest 
part of the atmosphere, describes, after reflection, aline, which is 
in the plane of the sensible horizon. 

CCLXXXV. A Natural Day is the time the 
Sun takes in passing from the meridian of any- 
place, till it comes round to the same meridian 
again ; but the natural days are not equal to 
each other ; and the Equation of Time, is the 
difference between the mean length of the natu- 
ral day, or 24 hours, and the length of any single 
day measured by the Sun's apparent motion, or 
between mean time and apparent time. 

Obs. 1. For any natural day is the time in which the earth per- 
forms one revolution on its axis, and such a portion of a second 
revolution as is equal to the Sun's increments of right ascension 
for that dav ; but the Sun's daily increments of right ascension 
are unequal ; therefore the additional portion of the second revo- 
lution will sometimes be greater and sometimes less, and conse- 
quently, the times in which the natural days are completed will 
be unequal. 

2. If the Sun were to move uniformly round the equinoctial in 
the same time in which it appears to describe the ecliptic, its 
apparent daily motion would be a measure of mean time. For 
the natural days in that ense being liable to no variation, either 
from the inclination of the Sun's apparent orbit, or the irregularity 
of its motion, must be equal. 

751. What effect does the atmosphere have on the Sun's rays 1 752. 
Why do we sep the ? un or a star b< : i'ere it is really above the horizon, 
at rising; and after t is below the horizon, at sett ng ? 753. What is a 
Natural Day? 754. What is the Equation of Time! 155. What is 
meant by Mean time and Apparent tune? 



r&ONOMY. 

Of the Sun. 

CCLXXXYI. The Sun is a spherical body, 
situated near the centre of gravity of the system 
of planets of which our Earth is one. It is the 
largest body yet known in the universe, its mean 
diameter beins: S87,000 miles, or about 112 times 
the mean diameter of the Earth ; and it revolves 
on its axis in about 25 days 10 hours. 

Ob?. I. From several phenomena it is concluded that the Sun 
is environed by an atmosphere, which extends to a considerable 
distance from its surface. It seems probable also that the solar 
light and heat are created by gaseous combustion. 

The direct light of the Sun is calculated by Euler to be equal to 
that of 6500 candles placed at the distance of one foot from the 
object or the eye ; while that of the Moon is calculated to be equal 
to the light of one candle at 7h feet distance; that of Venus, at 
421 feet ; and that of Jupiter, at 1C20 feet. Consequently the 
Sun would appear like Jupiter at 130,000 times its present dis- 
tance. 

2. The period of the Sun's revolution about its axis has been 
determined by means of several dark spots of various figures, 
which may commonly be seen with telescopes ; in the same man- 
ner have the periods of the revolution of Mars. Venus, and Jupiter, 
about their axes been determined ; whence it is inferred that this 
motion is general, and belongs to all the planets. 

3. If one of these spots appears upon the eastern limb or edge 
of the Sun's disk, or Jface, it moves from that edge towards the 
western edge in about 13| days, where the spot disappears ; and 
in about 13£ days more, it is seen a^ain upon the eastern edge j 
and so continues to go round, completing its revolution in 27 days ; 
during one-half of which time we see it on the disk of the Sun; 
ami during the other half it disappears. 

4. In the year 1779, there was a spot on the Sun, which was 
large enough to be seen by the naked eye. It was divided i to 

arts, and must have been 50,000 miles in diameter. Dr. 

chel supposed, that the spots in the Sun are mountains on 

in surface, which he thought may be more than 300 miles high. 

He examined the Sun with several powers from 90 to 500, and it 

756. Whal >un ; and bow is the c'un situated ? 

757. What is tiip diameter of the Sim ; ami the time of its rotation on us 

in -lie cm surrounded b v an atmosphere 1 759. 

pposed to creat the light and heat of the Sun! 7G0. What is 

ilculation of Euler, in regard to the comparative light of the Hun ? 

period of the 6un ; .s rotation been determined 1 



THE SUN. 221 

appeared, that the black spots are the opaque ground or body of 
the Sun; and that the luminous part is an atmosphere, which, 
beinsr intercepted or broken, gives us a glimpse of the Sun 
itself. 

5. He concludes, that the Sun has a very extensive atmosphere, 
which consists of elastic fluids that are more or less lucid and 
transparent ; and of which the lucid ones furnish us with light. 
This atmosphere, he thinks, is not less than 1843, nor more than 
2765 miles m height ; and he supposed that the density of the 
luminous solar clouds need not be greater than that of our aurora 
borealis, to produce the effects with which we are acquainted. 

6. The quantity of matter in the Sun, is to that in Jupiter nearly 
as 1067 to l, and the distance of that planet from the Sun. is in the 
same ratio to the Sun's semi-diameter; consequent!}', the centre 
of gravity of the Sun and Jupiter is nearlv in the superficies of the 
Sun. 

7. By the same method of calculation, it will be found, that 
the common centre of gravity of all the planets can not be more 
than the length of the soiar diameter distant from the centre of 
the San. 

8. The Sun's diameter being nearly equal to 112 diameters of 
the Earth, its cubic magnitude, therefore, must exceed that of the 
Earth by 1,406.550 times ; but its mass or quantity of matter is 
only 329,620 times that of the Earth ; therefore, the Sun would 
move but one foot by the action of the Earth, while the Earth 
would move 329,620 feet by the action of the Sun. 

9. lis similarity to the* other globes of the soiar system, in 
solidity, atmosphere, surface diversified with mountains, and 
valleys, and rotation on axis, lead us to suppose, that it is most 
probably inhabited, like the rest of the planets, by beings whose 
organs are adapted to their peculiar circumstances.* 

762. What is said of the spots seen upon the surface of the Sun 1 
763. What was Dr. Herschel's opinion respecting the heighiof the solar 
atmosphere ] 764. What are the relative quantities of matter in the Sun 
and Jupiter? 765. How is the centre of gravity of the planetary system 
shown to be w.thin the body of the £un? 766. How much does tho 
magnitude of the Sun exceed that of the Earth ? 767. What is the pro- 
band. ty of the fc'un's being inhabited? 



J * Our author, in this observation, has adopted the language of the 
celebrated : r. Herschel, with as much confidence as if the question of 
the planets being inhabited had been established beyond a doubt by that 
renowned astronomer. The ed tor, however, is not quite so desirous of 
multiplying the points of analogy, as demonstrative arguments of so 
improbable a position asthatof the Sun's being inhabited ; and the argu- 
ments which were so strenuously advanced by the learned astronomer 
in the Philosophical Transactions, towards the end of the last century, 
have been subsequently refuted by his own experiments and observa- 
tions published in the same series in the year 1600, but without the 
discrepancy being noticed. 

19* 



ASTRONOMY. 

li\ ii we are to consider all Hghl as analogous, from its equal 

rs on the eye, it seems likely that the light and heat of the 

Tasioned by the combustion of gas in the upper regions 

atmosphere, not dissimilar perhaps, to the combustion of 

carbureted hydrogen gas. with which we have lately illuminated 

our streets and houses. The simultaneous production of light and 

s in truth, in all cases to be produced by the combustion 

of ?as. 

11. Dr. Kerschel conceives the Sun and planets to have a 
gmeral motion at the rate of the Earth's motion in its orbit, with 
relation to the fixed stars ; but at this rate, if the distance of the 
stars is 200.000 times that of the diameter of the Earth's orbit, 
the Sun would be 00,000 years in moving over the distance of the 
nearest fixed star. 



Of the Primary Planets. 

CCLXXXVIL The Primary Planets, or those 
which are considered with regard to the Sun as 
' the proper centre of their motions, are eleven in 
number, which, according to the order of their 
distances from the Sun, beginning with the 
nearest, are, Mercury, Venus, Earth, Mars, Vesta, 
Juno, Ceres, Pallas, Jupiter, Saturn, and Uranus 
or Herschel.* 

Obs. Fig. 76 represents the planets in their true propor- 
tions. 

76 q . What arc primary planet?, their names, and number 1 

* The four small telescopic planets, or Asteroids, Vesta, Juno, Cer^s, 

and Pallas, which have lately been discovered revolving between the 

orbits of Mars and Jupiter, are generally, though improperly, excluded 

,e number of primary planets. 

Mercury, Venus, Earth. Mars. Jupiter, and Paturn, are visible to the 

naked eye, and have been known from time immemorial. Ce es was 

. ered on the 1st of January, 13 M, by M. Piazzi, an ingenious Italian 

imer. Pallas was discovered accidentally on the 2>th of March, 

v Dr. Olbers, at Bremen, in Lower Saxony, while looking out for 

which it much resembled when viewed with the telescope. Juno 

was discovered by Mr. Harding, at the observatory of Libenthal, on the 

evening of th< 1st of September, 1804, while he was making acatalogue 

of all the stars which wore near the orbits of Ceres and Pallas. Vesta 

was discovered by i)r. Olbers, at Bremen, March 29th, 1807. 



ELEMENTS OF THE PLANETS. 223 

Fie. 76. 




ELEMENTS OF THE PLANETS. 

CCLXXXVIII. The Elements of a Planets 
Orbit comprise the longitude of the ascending 
node of the orbit ; the inclination of the plane 
of the orbit to the ecliptic ; the mean motion of 
the planet round the Sun ; the mean distance 
of the planet from the Sun, or, which is the 
same, the semi-transverse axis of its orbit ; the 
eccentricity ; the longitude of the perihelion ; 
and, the time when the planet is in the peri- 
helion. 

Definitions. 

Ohs. 1. The Sun being placed in the focus of the elliptical orbit 
of a planet, the plan t can not always be at the same distance from 
the Sun ; but will be farthest from it when in the extremity A 
of the greater axis, most distant from the focus S, in which the 
Sun is ; and nearest to it when in P, Fig. 77. 

The point A is termed the Higher Apsis, or the Aphelion; and 
the point P, the Lower Apsis, or the Perihelion. These points 
are constantly varying, and their motion in a century, is called the 
Secular motion. 

769. What are the Elements of a Planet's Orbit ? 770. How is the 
motion of a planet in an elliptical orbit illustrated by a diagram 1 



224 



ASTRONOMY. 




The distance between the centre 
of the ellipse C, and the Sun or 
focus S, is called Eccentricity. 

The greater axis A P, is the Line 
of Apsides. 

The straight line S E, drawn 
from the extremity of the less axis 
E C to the Sun, is the Mean Dis- 
tance of the planet from the Sun. 

The mean distance added to the 
eccentricity, is equal to the Aphe- 
lion Distance. 

And the mean distance less by 
the eccentricity, is equal to the 
Per ih elion Distance . 

When the Sun or Moon is at the 
greatest distance from the Earth, 
it is said to be in its Apogee; and 
when at the least distance, in its 
Perigee. 

2. A planet docs not proceed in its orbit with an equable motion ; 
but in such a manner, that a line drawn) from the Sun to the planet, 
describes an area always proportionate to the time. For example, 
suppose a planet to be in A, whence in a certain time it. arrives at 
B ; the space or area winch the line S A, called the Radius -vector, 
describes, is the trianele ASB. Supposing the planet to be in P, 
let the straight line SD be so drawn that the area PSB may be 
equal to the area ASB ; then the planet will move through the 
arcs Al> and PD in equal times, though these arcs are unequal, 
being - nearly to each other reciprocally as their distances from the 
Sun ; for the areas being- equal, the arc PD is as much in propor- 
tion greater than the arc Ad, as SP is less than SA. 

3. A planet's Anomaly is its distance from the aphelion. When 
a planet is supposed to move in a circle, in the centre of which is 
the San, the portion FG of the circle bears the same ratio to the 
whole circumference, that the time since the planet passed its 
aphelion does to the time of its whole revolution, and is termed 
the Mean Anomaly. If the elliptical orbit of a planet, be so 
divided that the area ASG shall have the same ratio to the whole 
ellipse ABED, which the time since the planet passed its aphelion, 
has to its whole period, then is the angle ASG the measure of the 
planet's distance from the aphelion, at the time the planet is in G; 

771. What is meant by the Aphelion, Perihelion, Eccentricity, and 
Line if Apsides? 772. What is understood by Secular motion 1 773. 
What are the Mean distance, Aphelion distance, and Perihelion, dis- 
tance? 771. What is signified by Apogee and Perigee ? 775. How can 

it be shown that a planet does not proceed in its orbit with an equable 
motion ? 776. What is the Radius-vector? 777. What is meant uy the 
term Anomaly? 



DEFINITIONS. 225 

this angle is called the True Anomaly. The difference between 
the mean anomaly, and the true anomaly, is called the Equation 
of the Centre. 

4. When the motion of a planet is reckoned from the eauinoctiai 
point, it is called its Longitude. There are tables of each planet's 
mean motion, and of the equation of its centre, with its relative 
distance from the Sun, and its latitude in any part of its orbit; 
whence its true place may be calculated at any time. 

5. Let SGbe a mean proportional between the semi-axis major 
CA, and the semi-axis minor CE ; ihen, when the planet comes 
to the point G or L the equation of its centre will be the greatest, 
and this Greatest Equation varies according to the eccentricity of 
the orbit. The points G and L may be found by observation, and 
as A lies equally between them, the aphelion may from them be 
determined. 

6. A planet's Ehngation, or its angular distance from the Sun, 
is an angle formed at the Earth by two lines, one drawn from the 
Earth to the Sun, and one from the Earth to the planet. 

7. A planet's Periodic Time, or the time it takes in a revolution 
about the Sun, is found by observing when it is in any point of 
its orbit, and after any number of revolutions, obecrvins - when it 
comes-to the same point again ; that interval of time divided by the 
number of revolutions, gives the time of one revolution This is 
called a Tropical Revolution ; bui as, while the planet is making this 
revolution, the equinoctial point is moved by the precession of the 
equinoxes, there will bean additional time required for the planet to 
move, before it will be with the same star it was with, when the for- 
mer observation was made. and. therefore, the Sidereal Revolution, 
or that which brings it a^ain to the same star, exceeds the tropical. 

8. An inferior planet is in its Inferior Conjunction, when it is 
nearer to the Earth than the Sun is; and in its Superior Con- 
junction when it is farther than the Sun is from the Earth, at the 
time of the conjunction.* 

9. A person situated on the Earth, will perceive that a planet 
will appear to move sometimes in the order of the signs, or Direct, 
and sometimes in a contrary direction, or Retrograde, and to be 
sometimes Stationary. An inferior planet's motion is direct 
through its superior conjunction, and retrograde through its 
inferior ; between which situations it is stationary. A superior 

778 What is understood bv the Mean Anomaly. True Anomaly, and 
Equation of the Centre ? 779. What is a planet's Elongation ? 780. 
What is the Periodic lime ) 781. What is the difference between the 
Tropical and the Sidereal Revolution ? 782. What is signified by Direct 
niot.on, and what by Retrograde motion 1 783. Why is a planet some- 
times Stationary? ' 78'. Is the motion of an inferior planet direct or 
retrograde, whilst passing nearest to the Earth? 



* Those planets which are nearer to the fun than the Earth is, are 
.called Inferior planets, as Mercury and Venus ; and those which are 
more remote are called Superior planets, as Mars, Jupiter, and SaUun. 



226 ASTRONOMY. 

planet is retrograde when in opposition to the Sun, and direct 
when in conjunction with.it: aud between those situations it is 
stationary. 

10. The } lanes or levels of the planets' orbits are variously in- 
clined to the plane of the ecliptic. The opposite points in which 
an orbit crosses the ecliptic, are called Nodes ; that at which the 
planet rises north of the ecliptic, is called the Ascending Node, 
and the other the Descending Node. The line which joins the 
nodes, passing through the Sun, is called the Line of the Nodes. 

CCLXXXIX. An inferior planet is at its 
greatest elongation, when a line drawn from the 
Earth through the planet, is a tangent to the 
orbit of the planet ; and the greatest elongation 
of an inferior planet on one side of the Sun is 
not always equal to that on the other, because 
of the ellipticity of the planet's orbit. 

Obs. 1. The inferior planets are never in opposition to the Sun ; 
nor are they ever in quadrature, that is, 90° in the celestial sphere 
distant from the Sun. Hence these planets never appear far from 
the Sun. 

2. The inferior planets are invisible in their inferior conjunction ; 
their whole disk is illuminated, when they are in their superior 
conjunction ; and they are more or less illuminated, as they are 
nearer or farther from their superior conjunction. 

3. If an inferior planet is in or near one of its nodes at the time 
of its inferior conjunction it will appear as a dark spot en the disk 
of the Sun; which shows that the inferior planets receive their 
light from the Sun. 

CCXC. The superior planets are sometimes 
in conjunction with the Sun, sometimes in op- 
position, and sometimes in quadrature. And, 
the apparent diameter of a superior planet is 
greatest when the planet is in opposition ; for, 
when the planet is in conjunction, its distance 

785. When is the motion of a superior planet retrograde. &c. 1 786. 

What are the Nodes 7 7SJ, What is the Ascending Node ?— whnt the 

Descending Node I— and, what the Line of No ■ hy are the 

hto two classes, called Inferior and Superior 9 

■ plan t at ts greatest el mgation 1 79';. What 

I of the apparent d. stance of the inferior planets from the fcttn, the 

visibility of their disks, and their appearing sometimes as a dark spot on 

the disk of the ;Sun } 



MERCURY. 221 

from the Earth is greater than when it is in 
opposition, by the diameter of the Earth's orbit. 

Obs. 1. The apparent velocity of an inferior planet is greatest 
at the conjunction, and of a superior planet, at the conjunction and 
opposition. 

2. Of the superior planets, Mars sometimes appears roand, and 
sometimes gibbous ; Jupiter, Saturn, and Uranus, always exhibit 
a circular disk. For when Mars is either in opposition or con- 
junction, its whole illuminated hemisphere is toward the Earth, 
but when the planet is in quadrature, some part of its illuminated 
disk is turned from the Earth, etc. 

Of Mercury. 

CCXCI. Mercury is the first planet in the 
solar system, and though small, it shines with 
much brilliancy, but on account of its nearness 
to the Sun, it is seldom visible to us. 

Diameter of Mercury, in miles, . . - 3.180 

Mean distance from the Sun, 36.663,800 

Eccentricity, 7,434,400 

Rotation on its axis, - - - - Od. 24h. 5ra. 28sec. 

Sidereal Revolution, - - - 87 23 15 44 

Longitude of ascending node, Jan. 1, 1801, - Is. 15° 57' 31" 
Longitude of the perihelion, Jan. 1, 1801, - - 2 14 21 47 
Heliocentric longitude, Jan. 1, 1825, - - 23 10 
Geocentric longitude, Jan. 1, 1825, - - - 10 15 
Greatest equation of the centre, - - - 32 40 

Inclination of orbit, 0701 

Mean hourly motion, - - - - - 000 10 
Secular motion of the node, - - - - 1 10 27 
Secular motion of the line of apsides, - - 1 33 44 

Obs. 1 . The greatest elongation of Mercury from the Sun being" 
but 23° 48' it is mostly above the horizon when the Sun is ; and, 
therefore, is seldom seen. When it is visible, it is in the East 
just before sun rise, or in the West soon after sun set, accordingly 
as its place follows or precedes that of the Sun. 

2. When viewed through a telescope, it appears with pnases 
similar to the moon. When in its inferior conjunction, if its lati- 

791 . What is remarked concerning the appearance of a superior planet 
in different parts of is orbit I 702 What is. the appearance of Mercury 1 
— its diameter 1 — mean distance from the Sun I— the period of its sidereal 
revolution 1 — greatest elongation ? — and, time of its being seen 7 






228 ASTRONOMY. 

tude be less than the semi -dm meter of the Sun, it passes over the 
Sun's face; this is called a Transit of Mercury. 

3. The density of the Sun's heat, which is in the same propor- 
tion as its light, is seven times as great in Mercury as with us ; so 
that water there, would be carried off in the shape of steam; for 
Newton fjttitd, by experiments with the thermometer, it appears, 
that a heat seven times greater than that of the Sun's beams in 
summer, will serve to make water boil. This however depends 
on the weight of the atmosphere of Mercury. 

Of Venus, 

CCXCII. Venus, the next planet in order after 
Mercury, is distinguished from the other planets 
by its peculiar lustre, surpassing in brightness all 
the other stars and planets, and being sometimes 
so brilliant as to be visible in the daytime and by 
the naked eye. Its light is of a beautiful white 
colour, and so considerable as sometimes to pro- 
ject a shadow. 

Diameter of Venus, in miles, 

Mean Distance from the Sun, - 

Eccentricity, - 

Rotation on its axis, 

Sidereal Revolution, ... 

Longitude of ascending node, Jan. 1, 1801, - 2s. 

Longitude of the perihelion, Jan. 1, 1801, - 4 8 37 1 

Heliocentric longitude, Jan. 1, 1S25, - - - 15 29 

Geocentric longitude, Jan. 1, 1325, - - 10 IS 51 

Greatest equation of the centre, - - - 47 20 

Inclination of orbit, 3 23 30 

Mean daily motion, I 36 8 

Secular motion of the node, - - - - 52 20 
Secular motion of the line of apsides, - - 1 19 2 

Obs. 1. The greatest elongation of Venus varies from 45° to 
47° 42'. When the elongation of this planet is 39° 44' between its 
inferior conjunction and greatest elongation, it appears brightest; 
for then, though the enlightened part of its disk is hut 53-200lhs of 

7 3. What is signified by tbe Transit of Mercury? 794. How much 
greater is the effect of the Sun's heat and Hght at Mercury than on the 
surface of our globe .' 796. What is said of the appearance and lustre of 
Willie I 7«j. What is its diameter 1 the period of its sidereal revolu- 
tod diurnal rotation 7— mean distance from the Sou I — and greatest 
el mgationl 7.7. When duet Venus appear brightest? 



7,690 

68,514,000 

492,000 

• Od. 23/i. 2i?/i. Oscc, 

224 16 49 11 

14° 52' 40" 



THE EARTH. 229 

a circle, it is so much nearer to the Earth than in its superior con- 
junction, when it appears with a perfect disk, that the deficiency 
of enlightened surface is more than compensated by the intense- 
ness of the lig-ht; — in that situation, Venus is frequently seen in 
full day-light by the unassisted eye. 

2. Like Mercury, it sometimes parses over the Sums disk and 
its transit has been applied to one of the most important problems 
in Astronomy, as by it. the true distances of the planets from the 
Sun have been determined. 

3. When Venus is to the west of the Sun. it rises before the 
Sun, and is called a Morning- Star ; this appearance continues 
about 290 days together ; when this plane is to he east of the 
Sun, it sets after the Sun. and is cal ed an Evening Star, for about 
the same period 290 days. Venus has a consider ble atmosphere, 
and some astronomers assert, tha; they have discovered mountains 
on its surface. 

Of the Earth. 

CCXC1II. The Earth is the third planet in the 
solar system. It revolves round the Sun between 
the orbits of Venus and Mars, and in its revolu- 
tion, is attended by one satellite or moon. The 
Earth appears indeed to its inhabitants by far the 
largest body in the universe. It undoubtedly 
surpasses in dimensions several of the planetary 
bodies of our system, but when compared with 
the larger planets, particularly with the Sun. it 
dwindles almost to insignificance. 

Diameter of the Earth, in miles, ... - 7,920 

Mean ; istance from the Sun, 95,000.000 

Eccentricity, 1,613,000 

Rotation on its axis, ..... Qd. 23h. 56m. 4sec. 

Side; eal Revolution,' - - - - 365 6 9 ll| 
Longitude of the perihelion, Jan. 1, 1801, - 3s. 9° 30' 5'' 

Longitude. Jan. 1, 1301, 3 10 9 13 

Greatest equation of the centre, - - - 1 55 30 
Inclination of axis, Jan. 1, 1801, • - - 66 3: 2 
Mean daily motion, 00598 

798. When is this planet a morning star; and when an evening star ? 
7P9. What is said of the transit of Venus 1 £00. What is said of the 
appearance of the Earth ? BOl. What is the diameter of the Earth? — 
its mean distance iron the Snn, ? — rotation on lis a_xis ? — and its sidereal 
re voir it 



230 ASTRONOMY. 

CCXCIY. In the daily revolution of the Earth 
on its axis, the centrifugal force diminishes the 
weight.of bodies more at the equator than in any- 
other place on the surface of the Earth, in the 
duplicate ratio of the semi-diameter to the cosine 
of the latitude of the place. 

06s. 1. As the Earth revolves upon its axi^, every place on its 
surface, except the two poles, describes a circle, the plane of which 
is perpendicular to the axis, and the radius of which is the distance 
of that surface from the axis. 

2. Whence a bod] at the equator has its centrifugal force asmuch 
greater than a body between it and the pole, as the radius of the 
circle of the equator is greater than that radius ; and universally, 
the centrifugal force at the equator, is to the centrifugal force at 
any other place on the surface of the Earth, as the semi-diameter 
of the Earth is to the cosine of the latitude of the place. And 
since it is manifest, that the gravity must he diminished as much 
as the centrifugal force is increased, the gravity of a body at the 
equator, is as much more diminished thai, that of a bodv at any 
other place on the Earth, as the semi-diameter of the lEarth is 
greater than the cosine of 1 he latitude of the place.* 

3. It is found by calculation from tin's principle that gravity at 
the equator is diminished by the centrifugal force, in the ratio oi 
233 to 239, and if the diurnal motion of the Earth round its axis 
vere about 17 times faster than it is, the centrifugal force would, 
at the equator, be equal to the power of gravity, and ail bodies 
there would entirely lose their weight. But if the Earth revolved 
still quicker than this, they would all fly off. 

4. Since a place in the equator describes a circle of 24.880 miles 
in 24 hours,l it is evident that the velocity with which that place 
moves, is at the rate of 17.3 miles per minute. The velocity in 
any parallel of latitude decreases in the proportion of the cosine 
of latitude to the radius. Thus for the latitude of London, say. as 
rad. : cos. 51° 30' :*. velocity of the equator : velocity of London; 



S 2. What effect d >es the centrifugal force have on the weight of ter- 
restrial bodies 7 • 3 What is the ascertained law in regard to the action 
of the contr iftigal force and th^ diminution of gravity 1 801. If the 
Earth's diurnal rotation were 17 t mes more rapid, what would he the 
effect on the weight of bodies 1 8 5. What, is the velocity with which 
anyplace moves each minute at the equator? 

* Or, more properly, the diminut ; on of gravity at. any place, in conse- 
• of the centrifugal force, is asmuch less at that place as the square 
of the cosine is 1< pa than the square of the radius. 

t In strictness, this should be 23h. 56m. 4s.. the time in which th& 
Eajlh perfor:. axis. 



THE EARTH. 231 

by logarithms, a? 10 00000 : 9.794150 :: 1.232040:1.026196 = 10.6 
miles ; that is, London moves about the axis of the Earth at the 
rate of more than 10| miles in a minute of time. 

CCXCV. The Earth is an oblate spheroid, 
elevated at the equator and depressed at the 
poles. 

06s. 1. Tt has heen found by observation, aided by calculation,. 
that a pendulum, shorter by 226 inches, is required to vibrate 
seconds at the equator than at the poles ; hut the length of pen- 
dulums vibrating in the same time are as the gravities of the 
places where they vibrate ; therefore the gravity at the poles is 
greater than at the equator. And it has been found by Sir Isaac 
Newton, that this difference of gravity is so much greater than 
would arise from the centrifugal force alone, that the ratio of the 
equatorial diameter of the Farth to the polar diameter, must be as 
230 to 229, according to which the equatorial diameter exceeds the 
polar by about 34 miles. This does not correspond with the latest 
and most accurate measurements, which give for the difference of 
the two diameters about 25 or 26 miles. 

2. Hence bodies near the poles are heavier than the same bodies 
towards the equator, because they are nearer the Earth's centre, 
where the whole force of the Earth's attraction is accumulated, 
and because their centrifugal force is le s on account of their 
diurnal motion being slower. For both these reasons, bodies car- 
ried from the poles towards the equator, gradually lose a part of 
their weight. 

3. And the degrees of latitude upon the Earths surface are 
longer at the poles than at the equator. For an arc of a meridian 
near the poles is less curved than near the equator; that is. it is 
an arc of a larger circle ; whence a degree measured upon that 
arc, must be greater than upon an arc of the same meridian at the 
equator. 

4. The tendency of a heavy body, on any part of the surface of 
the Earth between the poles and the equator, is not directly to- 
wards the centre, but towards some point between the centre' and 
the equator. 

CCXdVJ. The equinoctial points move in 
antecedent ia, or go backwards from east to west, 
contrary to the order of the signs. And this 
motion or retro-gradation of the equinoctial points 

BOB. Compute the velocity of Philadelphia 807. What is the shape of 
tlie Earth ; and how is ' the d ff'erence be- 

tween the equal:. oi the Earth 1 809. Why arQ 

bodies heavier at the poles" than at the equator ? 






ASTRONOMY. 

occasions the tropical year to be shorter than the 
periodical year. 

Obs. 1. It is found from observation that the equinoctial and 
ecliptic do not always intersect each other in the same points, but 
that the points of intersection change their place, moving from 
e ist to we*t, in a retrograde direction, while the inclination of the 
planes of the ecliptic and equinoctial remain nearly the same. This 
motion has been called the Precession of tlie Equinoxes," because 
it carries the equinoctial points in precedentia signa, or forward 
in regard to the signs." The term precession of the equinoxes is 
now regarded as improper in the sense in which it has generally 
been used, and the term Recession has been proposed in its stead ; 
the former being restricted to the 20 minutes 2u seconds of time 
by which the equinoxes precede continually the complete apparent 
revolution of the Sun in the ecliptic. 

2. [{, while the Sun moves in the order of the signs, the equi- 
noctial point moves in the contrary direction, it is manifest, tnat 
the Sun must arrive at the solstitial or equinoctial point from 
which it set out, before it arrives at the same place in the zodiac, 
or must complete the tropical year sooner than the periodical year. 
The tropical year is observed to be 365 days, 5 hours, 48 minutes, 
5H seconds ; and the periodical year, which i> the same as the 
sidereal, 365 days, 6 hours, 9 minutes, 11£ seconds. 

CCXCVII. The Recession of the equinoxes is 
caused by the action of the Sun and Moon on 
that excess of matter about the equatorial parts 
of the Earth, by which it differs from a perfect 
sphere, in being of the form of an oblate spheroid. 

Obs. 1. If the excess of matter at the equator be considered as 
a ring encompassing the Earth at any distance, as Saturn is en- 
compassed by its ring; if it be supposed that this ring moves 
round its centre, the same way in which the Moon moves round 
the Earth ; it is obvious that every point of this ring will be acted 
upon by the disturbing force of the Sun in the same manner as 
the Moon is acted upon. Particularly, the motion of the nodes 
of this ring, and consequently, of the whole ring which moves 
with these nodes, and its inclination to the plane in which its 
centre moves, will be affected in the same manner with the orbit 
of the Moon, whence, its nodes, when in syxigies, will renin in 
stationary, and its inclination will be greatest; but in all other 
situations the nodes will go backwards, and fastest of all when in 

Why is the periodical or sidereal y^ar longer than the tropicall 
811. How are the recession of the equinoxes explained 1 812. From 
what cause does the recession of the equinoxes arise. 



THE EARTH. 233 

the quadratures, at wnich time the inclination of the ring will be 
the least. This will be the case whatever be the thickness of the 
rins, or its distance from the centre. If this ring be supposed to 
adhere to the Earth, it is obvious that it will still have the mo- 
tions described above, and that, in this situation, the Earth itself 
must participate of these motions. 

2. Hence the axis of the Earth, being- perpendicular to the plane 
of the equator, changes there with its inclination to the plane of 
the ecliptic twice in every revolution of the Earth about the Sun. 
For instance, it increases whilst the Earth is moving from the 
solstitial to the equinoctial, and diminishes as much in its passage 
from the equinoctial to the solstitial points; which phenomenon 
is called the Nutation of the poles. 

3. The student should be aware that the recession of the 
equinoxes, which merely chancres the position of the stars with 
reference to the nodes of the Earth's orbit, is different from the 
progression of the line of Apsides, or of the Aphelion or Perihelion 
points at the rate of 1° 43' 10' r in a century, or round the ecliptic 
in 20,931 years. Sir Richard Phillips, deduces from this motion, 
and from the varied action of the Sun in both hemispheres, a 
theory, by which he accounts for the present aggregation of water 
in the southern hemisphere, and concludes, that the same aggre- 
gation will take place in the northern hemisphere, when the peri- 
helion has its utmost northern declination. 

CCXCVIIL That periodical flux and reflux, 
or regular elevations and depressions of the ocean 
which occur twice every day, are in general de- 
nominated Tides, and are produced by the action 
of the Moon, and of the Sun. but more particu- 
larly by that of the former, upon the waters of 
the ocean. 

fbs. l. Two tides occur every 24 hours, 50 minutes, and 28 
seconds, agreeing with the mean interval from the Moon's leaving 
the meridian of any place till it comes to the same meridian again. 
Or, io other words, it is high at any place every 12 hours, 25 
minutes, and 14 seconds. 

2. The tides have been always found to follow, periodically, the 
course of the Sun and Moon, the latter in particular; and hence 
it has been suspected, in all ao-es. that the tides were, in some way 
or other, produced by these luminaries. The connexion of ike 

813. What is the between the motion of the line of apsides 

and tie precession of the equinoxes? 814. What has Sir Richard Phil- 
lips deduced from the recession of the equinoxes, and from the varied 
action of the Sun in both hemispheres 1 81.5. What are the Tides ; and 
how are they produced 1 516. fiow often do the tides rise and fall ? 
20* 



2 A 



ASTRONOMY. 



ebbing and flowing of the sea with the movements of the Moon, 
Ilis been remarked and is mentioned even by writers of the greatest 
antiquity, as Homer, Aristotle, Herodotus, Diodorus Siculus, and 
others. 

o. The waters of the oc< an are most elevated in that spot over 
the meridian of which the Moon has just passed. After the Moon 
DCS arisen at any place, the tide begins to rise, soon after it has 
passed the meridian, the tide is highest, and then gradually sub- 
tintil the setting of the Moon, after which it begins to rise, 
and is high again when the Moon is on the opposite side of the 
Earth, and then falls again until it rises; the time, like that of 
lh Moon's rising, being about 50 minutes later everyday. 
Fie. 73. 




lUus. 1. Let A;;Ln, Fig. 78, be the Earth, and C its centre ; 
let the dotted circle PN represent a mass of water covering the 
surface of the Earth; let M, m, be the Moon ; S, s, the Sun in 
different situations. Because the power of gravity diminishes as 
the squares of the distances increase, the waters on the side of 
the Earth A are more attracted by the Moon at M, than the cen- 
tral parts of the Earth C, and the central parts are more attracted 
than th.p waters on the opposite side of the Earth at L; conse- 
quently the waters on the side L will be attracted less than the 
centre, or will recede from the centre. Therefore, while the Moon 
is at M. the waters will rise towards a and I on the opposite sides 
of the Earth A, L ; while, by the oblique attraction ot the Moon, 
the waters at P and N will be depressed.* 

817. In what manner do the tides follow the course ol the Moon? 

' Philosophers havo been at no small trouble and have invented various 
singular theories to account for the tides or elevation of the waters on 



THE TIDES. 235 

2. Or thus ; because the Moon and Earth are continually re- 
volving about their common centre of gravity, suppose a ; the 
points A j C. L. describing circles about this com moil centre in the 
same periodical times, the forces required to retain them in these 
circles, will be to each other as their distances from the centre 
aA, aC, clL. Consequently, the point L requires a greater force 
than C, and C than A, to retain it in its orbit. Now these points 
are retained in their respective circles by the Moon at M; and, 
consequently, the point L, which is most remote, and therefore 
requires the greatest force, is attracted the least, whilst A. the 
nearest point, is attracted the most. Thus, the water about A 
being attracted too much, and that about L too little, boih will 
have their gravity diminished by the action of the Moon, and will 
endeavour to leave the centre C ; while the waters at P and N, 
having their gravity increased by the same cause, will subside. 
Hence the form of the water on the surface of the Earth will 
become an oblong spheroid. 

3. This oval of waters keeps pace with the Moon in its monthly 
course round the Earth ; while the Earth, by its daily rotation 
about its axis, presents each part of its surface, to the direct action 
of the Moon, twice each day. and thus produces two floods and 
two ebbs. But because the Moon is in the mean time passing 
from east to west in its orbit, it comes to the meridian of any place 
later than it did the preceding day ; whence the two floods and' 
ebbs require nearly 25 hours to complete them. The tide is at the 
greatest height, not when the Moon is in the meridian, but some 
time afterwards, because the force by which the Moon raises the 
tide continues to act for some time after it has passed the meridian. 

4. As the Moon thus raises the water in one place, and depresses 
it in an other, the Sun does the same ; but in a much less degree, 
on account of the small ratio of the semi-diameter of the Earth to 
the distance o[ the Sun ; for, as it was shown of the Moon, that 
the force of the Sun by which it disturbs its motion is as the dis- 
tance of the Moon from the Earth to that of the Sun from the 
same. so. in this case, the force of the Sun to disturb the waters 
is as the semi-diameter of the Earth to the distance of the Sun, 
which ratio is very small. 

CCXCIX. The tides are greatest at the new 
and full moons, and least at the first and last 

818. How may the cause of the tides be explained and illustrated by a 
diagram 7 



the side of the Earth opposite to the Moon. A little refection must con- 
v ; nce the intelligent reader that the t des produced at any place when 
the Moon is on the opposite are simply a kind of returning 

stroke vibration, or effect resBlting from the influence of the Moon while 

irations of a string 
or a pendulum continue som. time alter the impulse iirst communicated. 



235 ASTRONOMY. 

quadratures, and the highest tides are near the 
time of the equinoxes. 

When the Moon is in conjunction or opposition with the Sun, 
as 31, m, S, the tides which each endeavours to raise are in the 
same place ; whereas, when the Moon is in the first or last quar- 
ter, he Sun being in the meridian when the Moon is in the horizon, 
as M, Q,, depresses the water where the Moon raises it ; whence 
the tide* are then the least of all. On the full and new moons, 
which happen about the equinoxes, when the luminaries are both 
in the equinoctial or near it, tiie titles are the greatest of all ; for 
fir>t,the two eminences of water are at the greatest distance from 
the poles, and hence the difference between ebb and flood is more 
sensible : for if those eminences were at the poles, it is obvious 
we should not perceive any tide at all ; secondly, the equatorial 
diameter of the Earth produced, passes through the Moon, which 
diameter is longer than any other, and consequently, there is 
greater disproportion between the distances of the zenith, centre, 
and nadir, from the centre of gravity of the Earth and Moon, in 
this situation, than in any other ; and thirdly, the water rising 
higher in the open seas, rushes to the shores with greater force, 
where being stopped, it rises higher stiil ; for it not only rises at 
the shores in proportion to the height it rises to in the open seas, 
but also according to the velocity with which it flows from thence 
a rainst the shore. The spring tides, which happen a little before 
the vernal and after the autumnal equinox, are the greatest of all, 
because the Sun is nearer the Earth in winter than in the summer. 

The Moon being nearest to the Earth when in perigee, its 
attraction must then be strongest, and the effect, or the elevation 
of the waters, greatest. For the same reason, the Earth being in 
its perihelion at the winter solstice, the Sun's power to produce 
tides is greatest at that time. 

CCC. When the Moon is in the northern 
hemisphere, it produces a greater tide while it is 
in the meridian above the horizon, than when 
it is in the meridian below it ; when in the 
southern hemisphere, the reverse. 

Obs. 1. For the like reason, when the Moon is in the southern 
signs, the greatest tides on the other side of the equator will be 
when it is below our horizon, and the least tides when it is above 
the horizon. 

I full moons? — and why 

I last quarters? — and why are they highest of all about 

the time of the equinoxes 1 820. Whatissaid< deflect of the 

Moon in producing tides, according as it. is in the northern and southern 

hemisphere? 



MARS. 237 

2. What has been said of the tides, must be understood upon 
the supposition, that the globe of the Earth is entirely covered 
with water to a considerable depth ; but continents which stop 
the tide, straits between them, islands, and the shallowness of 
the sea in some places, which are all impediments to the course 
of the water, cause many exceptions which, can only be explained 
from particular observations on the nature of tides at different 
places. 

Of Mars. 

CCCI. Mars is the planet which comes next 
to the Earth in order from the Sun, being the 
first superior planet in the system. 

Diameter of Mars, in miles, 

Mean distance from the Sun, 

Eccentricity, - - " 

Rotation on its axis, 

Sidereal revolution, 

Longitude of ascending node, Jan. 1, 1801, 

Longitude of the perihelion, Jan. 1, 1801, 

Heliocentric longitude, Jan. 1, 1825, 

Geocentric longitude, Jan. 1, 1825, - 

Greatest equation of the centre, 

Inclination of orbit, Jan. 1, 1801, 

Mean daily motion, 

Secular motion of the node, - 

Secular motion of the line of apsides, 

Obs. 1. When Mars is in opposition to the Sun, it is nearest to 
the Earth, being about five times nearer to us than when in con- 
junction ; of course, it will appear much larger at the former 
station, and so gradually diminish its apparent magnitude in 
passing from opposition to conjunction. It never shines with a 
bright light, but has a dull red appearance, whence it is concluded 
it has a dense atmosphere. It appears with different phases 
according to its position, though never horned, its face being 
always greater than a semicircle. 

2. Numerous and extensive spots or shades, like those on the 
Moon, are discovered on this planet. Cassini observed spots in 
both hemispheres, by the motion of which he determined that the 
diurnal revolution was performed in 24h. 40m., agreeing very 
nearly with the subsequent observations of Dr. Hersehel. The 
belts and cloudy appearances on this planet are found to change 

821. What is the diameter of Mars?— its mean distance from the Sun 1 
—diurnal rotation I— and sidereal revolution 1 £22. What is the appear- 
ance of Mars 1 823. What is said of the spots upon its disk ? 





. 


- 


4.200 


- 


- 


144,760.000 




- 


13,463.000 


Od. 


24ft. 39m. 21.3sec. 


686 


23 


30 


36 




Ii 


Is. 


18° 


i/ 


27'' 




11 


2 


24 


2 


- 


11 


9 


33 





- 


10 


15 


39 





_ 





10 


41 


27 


- 





1 


51 


7 


- 








31 


27 


- 








44 


42 


- 





1 


49 


52 



238 ASTRONOMY. 

their shape and arrangement very frequently. Dr. Herschel dis- 
covered two white circles round the poles of this planet, which he 
supposed tobe produced by the reflection of the Sun's light from 
the snow lying about those extremities. The magnitude and 
splendour oi those remarkable luminous spots, he found to be sub- 
ject to great variation; and this variation in their size and bright- 
ness he supposed to arise from the melting of the polar ice. 

3. The following particulars respecting Mars are given by Dr. 
Herschel, after long and accurate observations. 

The axis of Mars is inclined to the ecliptic 50° 42'. 

The node of the axis is in 17° 47' of Pisces, or lis. 17° 47'. 

The obliquity of the ecliptic on the globe of Mars, is 28° 42'. 

The point Aries on the martial ecliptic answers to our 19° 28' 
of Sagittarius, or 8s. 19° 28?. 

The figure of Mars is that of an oblate spheroid, whose equa- 
torial diameter is to the polar one as 1335 to 1272, or as 16 to 15 
nearly. 

The equatorial diameter of Mars, reduced to the mean distance 
of the Earth from the Sun, 9" 8'". 

And that planet has a considerable, but moderate atmosphere, 
so that its inhabitants, proDaoly, enjoy a situation, in many re- 
spects, similar to ours. 

Of Vesta. 

CCCIL Vesta, the next planet after Mars, in 
the order of distance from the Sun, is of a dusky- 
colour, and shines with a purer light than any 
of the other minor planets ; though it requires 
the assistance of a telescope to render it distinctly 
visible. Its apparent diameter is not quite half 
a second. 

Diameter of Vesta in miles, ... - 238 

Greatest distance from the Sun, - - 246.450,000 

Least distance, - 204,420,000 

Mean distance, ... - 225,435,000 

Eccentricity, - - 21,015,000 

Sidereal revolution, - 1335d. Ah. 55m. 12sec. 

Longitude of ascending node, Jan. 1, 1801, 0s. 7° 8' 4G" 

Longitude of the perihelion, Jan. 1, 1801, - 8 9 43 
Mean longitude, Jan. 1, 1801, - - 8 27 25 l 

Inclination of orbit, - - - -078 46 

824. What particulars respecting Mars are given by Dr. Herschel 1 



- 




1.425 






327.433,000 


- 




198.404.000 






262.922.000 


- 




04.516,000 


1590d 


. 23h. 57m. 7sec. 




5s 

I 


21° 6' 38" 

23 13 41 


- 


9 


20 30 52 







13 3 23 



ASTEROIDS^ 23$ 

Of Juno or Harding. 
CCCIIL Juno is the next planet after Vesta, in 
the order of distance from the Sun. This planet 
is of a reddish colour, and appears sometimes 
very brilliant. 

Diameter of Juno, in miles. 

Greatest distance from the Sun, - 

Least distance, - 

Mean distance, - 

Eccentricity, - 

Sidereal revolution, 

Longitude of ascending node. Jan. 1, 1801 : 

Longitude of the perihelion, Jan. 1, ISOlj 

Mean longitude, Jan. 1. 1301, 

Inclination of orbit, - 

Of Ceres or Piazza. 
CCCIV. Ceres is the next planet, after Juno, 
in the order of distance from the Sun. Tt appears 
of a ruddy colour ; though perfectly invisible to 
the naked eye ; and it seems to be surrounded 
by an extensive and dense atmosphere. 

Diameter of Ceres, in miles, --. 1.624 

Greatest distance from the Sun, - - 233.502.000 

Least distance, . - - 242,306.000 

Mean distance, ----- 262.904,000 

Eccentricity, - 20.593,000 

Sidereal -evolution, - - 1631c?. \.2h. 56m. lOsec. 

Longitude of ascending node. Jan. 1, 1801, 0s. 10° c 7' 34" 

Longitude of the perihelion, Jan. 1, 1801, 4 26 39 39 

Mean longitude, Jan. 1, 1801, - - 8 24 45 10 

Inclination of orbit, - - - 10 37 34 

Obs. There is considerable uncertainty respecting the diameter 
of this planet ; Dr. Herschel has ascertained the real diameter to 
be only 163 miles; while, according- to the German astronomer. 3Ir. 
Schroeter. it is about 1624 miles, as given above. The same thing- 
may be observed with respect to the diameters of the other minor 
planets. 

Of Pallas. 
CCCV. Pallas is the eighth planet in the 
order of distance from the Sun ; appears less 



240 ASTRONOMY. 

rudely than Ceres, but its light exhibits greater 
variations ; and the height of its atmosphere, 
according to Schroeter, is 468 miles. Schroeter 
found the real diameter of this planet to be about 
80 miles ; but, according to Dr. Herschel, it is 
about 2099 miles. 

Greatest distance of Pallas from the Surij - 327,438,000 

Least distance, - 198,404,000 

Mean distance, - • - - - 262,922,000 

Eccentricity, - . . 64,517,000 

Sidereal revolution, - - 1681c/. \7h. 0m. 58sec. 

Longitude of ascending node, Jan. 1, 1801, 5s. 22° 32' 36" 

Longitude of the perihelion, Jan. 1, 1801, - 4 1 14 1 

Mean longitude, Jan. 1, 1801, - - 8 12 37 2 

Inclination of orbit, - - - 34 37 8 

Obs. These anomalous bodies, so unlike the other primary- 
planets, Dr. Herschel has denominated Asteroids. Probably they 
are the fragments of some comet ; or perhaps other similar bodies 
abound in the solar system, though they have hitaerto, from their 
smailness or darkness, escaped observation. Dr. Olbers conjec- 
tures that these recently discovered planets might be fragments 
of some very large planet, which once revolved between the orbits 
of Mars and Jupiter, and which had been burst asunder by some 
internal convulsion. 

Of Jupiter. 

CCCVI. Jupiter, the ninth planet in the order 
of distance from the Sun, is the largest planet in 
the solar system ; shines with a beautiful white 
lustre, and appears nearly as large as Venus. 

Diameter of Jupiter, in miles, ... 91,000 

Mean distance from the Sun, - - - 492,284,000 

Eccentricity, .... 23,810,000 

Rotation on its axis, - - Od. $h. 55m. 49. 7sec. 

Sidereal revolution, - - 4332 14 13 41 

825. What are the names, appearances, diameters, <fec. of the newly 

planets, or a 26. What is said of the orison of the 

new planets I 27. What is said of the sizeand appearance of Jupiter? 

Vhat is the diameter of this planet ?— its diurnal rotation?— sidereal 

re volution 7— mean distance. &c. ? 






35. 


8° 25' 


34" 





11 


8 


35 





4 


8 


54 





4 


8 


5 





o 


29 


25 





1 


18 


47 











59 








57 


13 





1 


34 


34 



JUPITER. 241 

Longitude of ascending node, Jan. 1, 1801, 
Longitude of the perihelion, Jan. 1, 1801, 
Heliocentric Jougitude, Jan. 1, 1325, 
Geocentric longitude, Jan. 1, 1825, - 
Greatest equation of the centre, 
Inclination of orbit, Jan. 1, 1801, 
Mean daily motion, - 

Secular motion of the node, 
Secular motion of the line of apsides, 

Obs. l. The form of Jupiter, like that of the Earth, is an oblate 
spheroid, the equatorial diameter being to the polar as 14 to 13 ; 
and the difference of the two diameters about 6000 miles. Its 
axis is so nearly perpendicular to the plane of its orbit, that it 
can not have any sensible variation of seasons ; so that the polar 
regions of Jupiter must necessarily be subject to perpetual winter, 
and the equatorial regions to perpetual summer. 

Jupiter, like Mars, appears largest when in opposition to the 
Sun ;— when it is to the west of the Sun, it is a morning- star ; — 
and when to the east of it, an evening star. 

2. When viewed through a good telescope, Jupiter is observed 
to be surrounded by many faint or cloudy substances, subject to fre- 
quent changes in their situation and appearance, called Zones or 
Belts, which are not only parallel to each other, but, in general, 
parallel to the planet's equator ; as they are seen extending across 
the disk, in lines parallel to the equator. These belts are subject 
to considerable variation in number, distance, position, and mag- 
nitude ; and are sometimes of a regular form, sometimes inter- 
rupted and broken, and sometimes not at all to be seen. Bright 
and dark spots are also frequently to be seen in the belts ; and 
■when a belt vanishes, the contiguous spots disappear with it. 

Different opinions have been entertained by astronomers respect- 
ing the cause of these belts and spots. Some consider them as the 
effect of changes in the atmosphere that surrounds Jupiter; and 
some have regarded them as clouds or openings in the atmosphere 
of that planet, or rather as clouds drawn into parallel strata by the 
rapidity of the diurnal rotation ; while others imagine them to be 
of a more permanent nature, and the indications of great physical 
revolutions, which are perpetually agitating and changing the sur- 
face of the planet. 

3. By directing a telescope to the planet Jupiter, four small 
stars are discovered accompanying the planet, ranged nearly in a 
straight line parallel to the plane of the belts. These small stars 
are the moons or satellites of Jupiter, which move round the planet 
in different periods, and at unequal distances from their primary. 
These four satellites, which were discovered by Galileo, in 1610, 
soon after he had invented the telescope, may in general be seen 

829. What is said of the shape of Jupiter?— its faint cloudy appear- 
ances, or Belts'l— and of the number of its satellites 1 



242 ASTRONOMY. 

throiujh a telescope that magnifies 30 limes; and, indeed, some 
astronomers pretend to have seen the third and fourth with the 
naked eye. Their relative situation with regard to the planet, as 
well as to each other, is constantly changing. 

Of Saturn. 

CCCVII. Saturn, the next planet in the solar 
system, is next in magnitude to Jnpiter ; but 
shines with a very feeble light, compared with 
that of Jupiter, — partly on account of its great 
distance from the Sun, and partly from its dull 
red colour. 

Diameter of Saturn, in miles, - - 78,000 

Mean distance from the Sun, - 906.633,000 

Eccentricity, - 49^000,000 

Rotation on its axis, - - Od. iOh. 16m.. 49sec. 

Sidereal Revolution, - - 10,758 23 16 34 

Longitude of ascending node, Jan. 1, 1801, - 3s. 22° 4' 27'- 
Longitude of the perihelion, Jan. 1, 1801, - 2 29 30 53 

Heliocentric longitude, Jan. 1, 1825, - 2 5 40 

Geocentric longitude, Jan. l, 1325, - 2 l 44 

Greatest equation of the centre, - 6 27 58 

Inclination of orbit, Jan. 1, 1801, - 2 30 13 

Mean daily motion, - - 2 6 

Secular motion of the node, - - 55 30 

Secular motion of the line of apsides, - l 55 47 

Ohs. 1. Saturn is a beautiful object for a good telescope ; having 
seven moons or satellites, certain obscure zones or belts appearing 
at times across its disk, similar to the belts of Jupiter; and what 
distinguishes this from all the other planets, is a large luminous 
ring, surrounding the body of the planet, without touching it — a 
phenomenon unique^ as far as we know, in the solar system. 
Saturn's Ring- is a thin, broad, flat circle, encompassing the planet 
a1 a distarj very similar to the horizon of an artificial 

surrounding the globe ; and as Saturn's axis is perpendicu- 
lar to the plane of the ring, if the globe be rectified for the pole, 
it will aptly represent the planet and the ring. 

2. This most singular phenomenon was discovered by the cele- 
brate soon after the invention of telescopes. He found 

that the ring is inclined by a constant angle of about 30° to the 
plane of Saturn's orbit, and therefore at nearly the same angle 

830. V diameter 1 — its mean distance from the Sun 7 — ' 

rotation on its axis ?- ami sidereal revolution 1 



CJRANTTS. 243 

[31° 23'] to our ecliptic, and hence always appearing to us ob- 
liquely. This ring, which casts a deep shadow upon the body of 
the planet, is sharply defined both in ns inner a .d outer edge, and 
appears more luminous than Saturn itself. Hence Dr. Herschel 
has concluded that it is not any shining fluid, or aurora borealis, 
as some have imagined ; but a "solid body, equal in density to the 
planet itself. 

3. The latter discoveries of Dr. Herschel have shown that what 
was supposed to be a single broad flat ring of Saturn, is in reality 
divided into two parts, situated exactly in the same plane, and re- 
volving about an axis perpendicular to that plane, in the same 
time that Saturn revolves on its axis,* so that, instead of one ring, 
there are two concentric nn^s. detached from each other, and from 
the body of the planet ; the inner ring being nearly three times as 
broad as the outer one. 

4. Tne following dimensions of this double ring were deter- 
mined by Dr. Herschel. 

Inner diameter of the smaller ring, - 146.345 miles. 

Outside diameter of the same. - 184.393 

Inner diameter of the larger ring, ... 190.24S 
Outside diameter of the same, ... 204,883 

Breadth of the inner ring, 20.0 JO 

Breadth of the outer ring, - 7,200 

Breadth of the vacant space, or dark zone, - - 2.S40 
Thickness of the ring, 1,000 

5. Saturn has an atmosphere extending to the ring; and the 
zones, or belts, are supposed to be obscurations in the planet's 
atmosphere, whichDr. Hersche, has observed to be of considerable 
density ; though the fixed stars have been seen through it between 
the ring and the body of the planet. 



Of 



Uranus. 



CCCVIII. Uranus, is the most remote planet 
yet discovered in the solar system ; its distance 
from the Earth being so great, that it can not he 
seen by the naked eye. except when the atmo- 
sphere is very clear, and then it appears like a 
star of the sixth magnitude ; nor can it be rea- 

831. What is said of Saturn's 2. To what distance from the 

planet do:s the atmosphere of Saturn extend ? 

* " Both the planet and its ring- turn round the same common axis ; but 
the ring seems to revolve somewhat slower, being lOh. 32m. 15.4 sec. in 
performing one revolution." 



5 


17 


38 19 


9 


16 


10 


9 


17 


23 





5 


21 7 








46 








42 








29 59 





1 


27 29 



244 ASTRONOMY. 

dily distinguished from a fixed star, with a less 
magnifying power than about 200. 

Diameter of Uranus, in mileSj - 35,000 

Mean distance from the Sun, - 1,812,414,000 

Eccentricity, ----- 85,052,000 

Rotation on its axis, - unknown. 

Sidereal revolution, - 30,688d. i7h. 6m. 2sec. 

Longitude of ascending node, Jan. 1, 1801, - 2s. 12° 55' 42' 

Longitude of the perihelion, Jan. 1, 1801, - 

Heliocentric longitude, Jan. 1, 1801, 

Geocentric longitude, Jan. 1, 1801, - 

Greatest equation of the centre, 

Inclination of orbit, - - - - 

Mean daily motion, - 

Secular motion of the nodes, 

Secular motion of the line of apsides, - 

Obs. 1. This planet had escaped the observations of the ancient 
astronomers, in consequence of its very great distance from the 
Sun and from the Earth, which makes it appear so small as to be 
seldom visible without the aid of a telescope. Flamslead, at the 
end of the seventeenth century, and Mayer and Le Monnier, in the 
eighteenth, had observed Uranus as a small star. But it was not 
till the 13th of March, 1781, that Dr. Herschel, while pursuing- a 
plan which he had formed of observing, with telescopes of his own 
construction, every part of the heavens, discovered, in the neigh- 
bourhood of r) Geminorum, the motion of this apparently small 
star ; and soon after, by carefully following the same, he ascer- 
tained it to be a true planet. In compliment to his royal patron, 
King George III., Dr. Herschel named the planet Georgium 
Sldus, or the Georgian Planet ; by the French astronomers, it 
was called Herschel, from the name of its indefatigable and truly 
great discoverer ; and Professor Bode, of Berlin, gave it the name 
of Uranus, who, according to the ancient Heathen Mythology, 
was the father of Saturn as Saturn was of Jupiter. It is now 
more generally known in this country by the name Uranvs. 

2. The Sun's diameter, as it would appear to a spectator in 
Uranus, is equal to twice the apparent diameter of Venus seen 
from the Earth ; and the intensity of the solar light and heat on 
this distant planet, is to that on the Earth, with regard to the in- 
fluence of the Sun's rays, as 276 to 100,000, or as 1 to 362 nearly. 

3. Dr. Hersdiel has discovered six satellites revolving round 
Uranus, in orbits which are nearly perpendicular to the plane of 

: Wli.u is sai'l of tho remoteness and appearance of Uranus 1 834. 
Wh.it is its diameter 7— mean distance from the Sun?- ami sioYreal re- 
volution? 835. When ami by whom was this planet discovered 1 
Wn'al is the relative intensity of the solar rays at the surface of this 
distant planet? 



SECONDARY tLAKETS. 215 

the ecliptic. Two of these satellites — (he second and fourth — ■ 
were discovered in 1787 ; the first and fifth in 1790; and the other 
two in 1794. They are altogether very minute objects; for although 
the planet itseifmay be seen, in general, with almost any telescope, 
yet its satellites can not be perceived without the sid of the most 
powerful instruments, and the concurrence of all other favourable 
circumstances. 

CCCIX. All the planets revolve round the Sun 
from west to east, or in the order of the signs ; 
and in the same direction do all the satellites re- 
volve round their primaries. The diurnal rota- 
tion of every planet which is known to have such 
a rotation, is also in the same uniform direction, 
— from west to east. 

Obs. 1. If the pl&nets Mercury, Venus, the Earth. Mars, Jupiter, 
and Saturn, be in conjunction at anytime; in the period of 230,000 
years they r will be very nearly in conjunction again : For. 

Revolutions. 
Mercury after making - - U 62577 in 8836185093921 

Venus" - - - 455122 8835595689448 

The Earth - - - 230000 3335940630000 

Mars - 143* 5946519500 

Jupiter - 23616 1444S 

Saturn - - - - 951 G 8835946558608 

2. An easy distinction between a planet and a fixed star is this ; 
the former shines with a steady light, but the latter is constantly 
twinkling. This twinkling or scintillation of a star is supposed 
hy some to be occasioned by the irregular progress of the li?ht 
from such distant bodies to the eye. Others imagine the twinkling 
of the fixed stars to arise from the atoms in the atmosphere con- 
tinually intervening. In the equatorial regions, the stars shine 
with a clear steady light like the planets. The true cause has 
not yet been discovered. 

Of the Secondary Planets. 

CCCX. The secondary planets are those which 
move round some primary planet, as their centre 
of motion, in the same manner as the primary 
planets move round the Sim. 

637. What is remarked concerning the six satellites of Uranus 1 633. 
In whni direction is the diurnal and annual revolution of the planets? 
839. What is an eatry distinction between a planet and fixed sous? 
21 



246 ASTRONOMY. 

The Earth is thus attended by one secondary planet, called the 
Moon ; Jupiter by four, Saturn by seven, and Herschel by six. 

( JCCXI. The motion of a secondary planet in 
its orbit is not nearly so uniform as that of a 
primary ; because though every secondary gra- 
vitates chiefly towards its primary as a centre, 
yet its motiou is much disturbed by the unequal 
action or influence of the Sun. 

Obs. The satellites of Jupiter, Saturn, and Uranus, are fre- 
quently eclipsed by their respective primaries ; and, when any 
satellite passes between the Sun and its primary, it eclipses the 
Sun. 

Of the Moon. 

CCCXIL After the Sun, the Moon is the most 
interesting to us, of all the celestial bodies, and 
is particularly distinguished by the periodical 
changes to which its figure and light are subject. 

Obs. 1 . Like all the primary planets, the Moon is a solid, opaque 
body, of a globular form ; and though comparatively a small body, 
it appears, in consequence of its nearness to us, almost as large as 
the Sun. Next to that luminary, if is indeed the most brilliant 
and beautiful orb that we observe in the heavens ; and as such, it 
has at all times been distinguished, both by the rude and by the 
most cultivated and civilized nations of the Earth. 

2. The Moon is a secondary planet to the Earth, round which 
it revolves once a mo.ith, at the same time attending its primary 
round the Sun. 

Def. 1. The Tropical Revolution signifies the complete revo- 
lution of 12 signs, performed round the Earth, and is sometimes 
called a Periodica! Month. 

The Sidereal Revolution signifies a completion of the motion 
to the same star, and is something longer than the tropical, on 
account of the Recession of the equinoxes. 

The Synodic Revolution is the time from one conjunction with 
th<" Sun. or one new-moon to an other; this exceeds t hie sidereal, 
and it will be found that in 29d. 12h. 44' 2". 8283 the Moon will be 
again in conjunction with the Sun, the Earth being at the distance 
of 29° G' 20 .2 from the place of the former conjunction. 

Whj is the motion of a secondary plan< t less uniform than that 
of a primary 1 811. What, is the Synodic revolul 



THE MOON. 247 

2. When the Moon is at its greatest distance from the Earth 
in its orbit, which is elliptical, or at its higher apsis, it is said to 
be in its Apogee; when at its least distance, or lower apsis. in its 
Perigee. . . 

3. When the Moon is in conjunction with the Sun, it is Aeff*- 
Moon ; when in opposition, it is Full-Moon ;— its conjunction and 
opposition are called by the common name Syzigies. 
Diameter of the Moon, in miles, - 2,160 
Mean distance from the Earth, ... - 236,400 

Eccentricity, 12,980 

Rotation on its axis, - - - 29d. 12>i. 44m. 2.33sec. 

Greatest equation of the centre, - - - - 6° 17' 9' 
Greatest inclination of the orbit, - - - - 5 13 6 
Least inclination of the orbit, - - - - 5 

Greatest apparent diameter, - - - - - 33 31 
Least apparent diameter, - - - - - 29 30 
Horizontal parallax, . - - 53' 52" to 61 32 

Mean tropical revolution, - - 27d. 7h. 43m. 4 sec. 

Mean sidereal revolution, - - - 27 7 43 11.5 
Mean synodic revolution, - - 29 12 44 2.53 

Sidereal revolution of the perigee, 3y. 312 11 11 39.4 
Sidereal revolution of the node, 18 223 7 13 17.7 
Mean diurnal motion of the perigee, 0° 6' 41' 

Mean diurnal motion of the node, - - - - 3 10 
Mean diurnal motion of the Moon in respect to the 

equinoxes, 13 10 35 

CCCXIII. The Moon at its conjunction, is 
invisible ; at its opposition, its whole disk is en- 
lightened ; at its quadratures it is half enlight- 
ened ; between the conjunction and quadrature, 
it is horned ; and between the quadrature and 
opposition, it is gibbous. 

Illus. In Fig. 79. S is the Sun, T the Earth, ABC. &c. the 
Moon in its orbit. One half of the Moon is always enlightened 
by the Sun. At A, the Moon is between the Earth and Sun. it is 
then new ; and is invisible as represented at a; at B the enlight- 
ened part x z is turned to the Earth, and it appears horned as at 
b; at C the half of the enlightened side is turned to the Earth, 
and it appears a half moon as at c; at D the part x z is turned to 
the Earth, and it appears as at d; and at E the whole of the en- 

842. What is meant by the Moon's Syzigies 7 843. What is the diame- 
ter of the Moon 1 — its mean distance from the Earth ? — and its sidereal 
revolution? 854. What is the appearance of the Moon at the conjunc- 
tion, opposition, quadratures, &c. ? 






ASTRONOMY. 

lightened part of the Moon is turned to the Earth, and we have 
i as at r. ^ As it passes through the rest of ti e orbit, it 
on the same phases as before, but in a contrary order. 



full- 
puts 





Obs. The Earth must be a satellite to the Moon, and subject to 
the same changes, but more than 13 times larger than the Moon 
appears to us. At new-moon to us, the Earth appears full to the 
Moon, etc. 

CCCXIV. The time of the Moon's rotation 
on its axis is equal to the mean time of its revo- 
lution in its orbit round the Earth ; which is 
inferred from the fact that the Moon always has 
nearly the same side towards the Earth. 

Ohs. I. It appears to be a very remarkable circumstance, as far 
hitherto been observed, that the satellites are subject to the 
same law that our Moon obeys, that is, they revolve ou their axis 
in the same time in which they revolve about their primaries. 
For, in consequence of observing periodical changes in the inten- 
sity of the light of Jupiter's satellites, Dr. Herschel inferred ihat 
volveon tbeirajtes, and that the periods of their rotations are 
al to the times of their revolutions round Jupiter. 
Dr. Herschel discovered also, that the seventh satellite of Saturn 
le law ; and hence he thought it natural to 

■ )i the Moon illustrated by means of a diagram 

or flgui •.:>< times invisible when it is above 

• en the Moon's rotation 
LXis, and the period of its revolution round the Earth? 



THE MOON. 249 

conclude that all the secondary planets mi^ht be governed by the 
same law to which those of Jupiter, the Moon, and the seventh 
Saturnian satellite, are subject. This theory seems also to be 
confirmed by certain other observations made by that celebrated 
astronomer. 

2. Though the year is of the same absolute length, both to the 
Earth and Moon, yet the number of days in each is very different ; 
the former having 365? natural days, but the latter only about 
12 and seven-nineteenths, every day and night in the Moon being 
as long as 29| on the Earth. 

CCCXV. The Moon appears to have two 
Ubrations, one upon a line perpendicular to its 
axis, called its Libration in Latitude ; the other 
upon its axis, called its Libration in Longitude. 

Obs. 1. This appears from observation ; some small portions of 
the surface of the Moon being visible in some parts, and invisible 
in other parts, of its orbit; that is, in consequence of its libration 
in latitude we sometimes see one pole and sometimes the other. 
And by the libration in longitude, more of the western limb is 
sometimes seen, and at others more of the eastern. The inclina- 
tion of its axis to the orbit is 36°. 

2. The libration in latitude arises from the obliquity or inclina- 
tion of the axis to the orbit ; and the libration in longitude, from 
the eccentric form of the orbit. A Diurnal Libration also takes 
place ; at rising, a part of the western edge is seen, which is invi- 
sible at setting ; and the contrary is ob erved with respect to the 
eastern edge. This is occasioned by the change of place of the 
spectator, on account of the Earth's rotation. There is also an 
other kind of libration, for the explanation of which, however, the 
reader is referred to Gregory's Elements of Astronomy. 

CCCXVI. The nearer the Moon is to its 
syzigies* the greater is its velocity ; and the 
nearer it is to its quadratures, the slower it 
moves. 

CCCXVII. When the Earth is in its perihe- 
lion, the periodical time of the Moon is the great- 

848. How many clays complete a lunar year, or year at the Moon ? 8-19. 
How many of our days are equal to one day at the Moon ? ^50. To how 
many kinds of librations is the Moon subject? — how are they distin- 
guished and denominated 1 — and by what are they produced ? 

* The line joining the centres of the Sun. Earth, and Moon, at the new 
and full moon, is called the Line of Syzigies. 



250 ASTRONOMY. 

est ; when the Earth is in its aphelion, the 
periodical time of the Moon is the least. 

Obs. 1. Since all the irregularities of the Moon's motion pro- 
ceed from the action of the Sim, it follows, that where the action 
of the Sun is greatest, the irregularities arising from it will be 
greatest too. But the nearer the Earth is to the Sun. the greater 
will he the action of the Sun upon the Moon ; and the more the 
Moon tends towards the Sun, the less will it tend towards the 
Earth. When, therefore, the Earth is at the perihelion, and con- 
mtly at its ice from the Sun, the action of the Sun 

upon the Moon will be greatest, and destroy more of its tendency 
towards the Earth than at any other distance. Therefore, when 
the Karth is at the perihelion, the Moon will describe a la 
orbit about the Earth, than when the Earth is at any other dis- 
tance from the Sun, and, consequently, its periodical time will 
then be the longest. But the Earth is at its perihelion in the 
winter, and, consequently, then the Moon will describe the outer- 
most circle about the Earth, and its p riodical time will be the 
longest; whicli agrees with observatioi . For the same reason, 
when the Earth is in its aphelion, tl e tendency of the Moon 
towards the Earth will be the greatest, and, consequently, its 
periodical time the least. And in this case, which will be in the 
summer, it will describe the innermost circle about the Earth. 

2. The radius-vector of the Moon describes equal areas in equal 
times only at the syzigies and quadratures, and deviates from this 
law the farthest when in the octants ; and the orbit is more curved 
in the quadratures, and less in the syzigies, than it would be if the 
Moon were only attracted by the Earth. 

CCCXVIII. The eccentricity of the Moan's 
orbit is varied in every revolution of the Moon, 
and is greatest when the Moon is in syzigy, and 
least when it is in quadrature ; and the orbit is 
most of all eccentrical when the line of the apses 
is in the syzigies, and least of all eccentrical when 
this line is in the quadratures. 

Obs. 1. And if we compare several revolutions of the Moon 

her, we shall find, that when the line of the apses is in the 

ntricity will be the greatest of all, because in that 

the tendency which the Moon has 

•.he apses, and that which it has in the oppo- 

51. ". | ■,; part the irregularities of the 

v which they are produced 3 852. 
What is said respecting the eccentricity of the Moon's orbit 1 



THE MOON. 251 

site one is the greatest of all* whereas, when the line of the apses 
is in the quadrature, this difference is the least, and therefore the 
lunar eccentricity will be so too. 

2. When the gravity of the Moon towards the Earth increases 
or decreases too fast, the eccentricity of the orbit will increase ; 
and when the gravity increases or diminishes too slow the eccen- 
tricity of the orbit will decrease ; and the orbit itself will approach 
nearer to the form of a circle. 

3. All the irregularities of the Moon are greater when the Earth 
is in its perihelion, than when it is in its aphelion, because the 
effect of the Sun s action, whereby they are produced, is inversely 
as the square of its distance from the Earth. They are also greater 
when the Moon is in conjunction witli the Sun, than in opposition, 
for the same reason ; for the Earth and Moou. taken together, are 
nearer the Sun in the former situation of the Moon, than they are 
in the latter. 

CCCXIX. The Moon is a solid opaque globu- 
lar body, and its surface is covered or diversified 
with innumerable irregularities. 

Obs. 1. If the surface of the Moon were smooth and polished 
like a mirror, the image of the Sun, which in certain situations 
would be reflected to us, would only appear like a very bright 
point ; and the Moon would otherwise be wholly invisible to i:s. 
But the surface of the Moon is far from being smooth, and its 
inequalities reflect the Son'* light in all directions ; hence we see 
all those parts of that surface which are illuminated by the Sun, 
and which are at the same time within the direction of our sight. 

Even to the naked eye, some of the irregularities of the Mooirs 
surface appear like less bright or darkish spots, which appearance 
has suggested the vulgar idea of the Moon's having a face with 
eyes,&c. and those parts have likewise been imagined to be seas. 
But when viewed through a tolerably good telescope, the surface 
of the Moon appears covered with vast irregularities, and wonder- 
fully diversified with ; kSj mountains, hills, valleys, hol- 
lows, and pits, of infinite variety. 

The spots which are seen on the surface of the Moon are not 
mere variations of colour, or of light and shade, but they arise 
from real inequalities of surface, such as mountains, vales, pits, 
hollows, ridges, &e. which is evidently proved by the tria' 
shadows, which they cast in a certain direction, according t 
relative situation of the Sun. and by the elevated parts becoming 
illuminated by the Sun before the lower parts. The pans so 
situated as best to reflect the Sun's light, appear most lucid ; 
while others, obscured by shadows projected from eminences, 

853. What is the appearance of the Moon 1 854. How is it proved that 
the Moon's surface is diversified with mountains and vaiiey-s ?- 



252 ASTRONOMY. 

appear comparatively dark. When, at any other time than at the 
full of the Moon, the line which separates the enlightened from 
the dark part on the disk, is turned towards us, we can see it 
through a telescope, not as a regular line, but notched or jagged, 
and full of irregularities, especially some small bright dots or 
ridges at a little distance from the "illuminated part of the disk, 
which are the tons of mountains and other elevated parts, that are 
illuminated by the Sun, while their lower parts can not receive 
the solar rays. 

2. From the attempts of astronomers to measure the height of 
the lunar mountains by means of their shadows, various results, 
it seems, have been obtained. Galileo, the author of the discovery, 
determined the height of some of them to be nearly nine miles ; 
and, according to Recciolus, the summit of the hill, called St. 
Catherine's, is nearly eight miles in height; but Dr. Keill, in his 
Astronomical Lectures, has calculated the height of the same moun- 
tain to be about nine miles. Later astronomers, by being- pos- 
sessed of more accurate means of measurement are not inclined 
to allow of so great an elevation to any of the lunar mountains. 
Dr. Herschel investigated the height of a great many ; and he 
has concluded that, with a very few exceptions, the lunar moun- 
tains in general do not exceed a mile and a half in their perpen- 
dicular elevation. 

M. Schroeter, of the Royal Society of Gottingen, in the year 
17 92, seems to have taken particular care in investigating the 
truth of this matter. According to him, the surface of the Moon 
appears to be much more unequal than that of our Farth ; and 
these inequalities have great variety both in form and magnitude. 
There are large irregular plains, on which are observed long and 
narrow strata of hills, running in a serpentine direction ; some of 
the mountains form extensive chains, others, which are in gene- 
ral the highest, stand alone, and are of a conical shape ; some 
have craters ; others form a circular ring inclosing a plane. Ac- 
cording to Mr Schroeter, the height of the mountain distinguished 
by the name Leibnitz, is not less than 25,000 feet ; so that, taking 1 
into consideration the relative magnitudes of the Earth and Moon, 
this lunar mountain is considerably higher than any of the moun- 
tains on our globe. 

Mr. Schroeter has also at different times measured the Moon's 
its, the craters of which are generally circular, and surrounded 
y an annular bank of hills. They are remarkable for their width, 
many of them being from 4 to 15 geographical miles in diameter ; — 
some are not deeper than the level of the Moon's surface ; others 
are 9,000, 12,000, 15,000, and even 24,009 feet m depth. The depth 

\v"'i.!t [» the description given of the spots and irregularities of the 

What is said of the height of the lunar nioun- 

.". What is the result of Dr. Herschel's observations on the 

altitude of the lunar mountains. 858. What has Mr. Schroeter ascer- 

ta n J in regard to this subject! 



I 



ECLIPSES, 253 

of one which Mr. Schroeter called Bernoulli^ was found to be more 
than 18,000 feet. 

3. A bright speck or two, or even three, have sometimes been 
observed on the dark part of the Moon's disk, and so far from the 
illuminated part as not to depend upon the rays of light from the 
Sun. Those lucid spots have been conjectured to be volcanoes, or 
burning mountains, which after a certain time become extinct and 
disappear. Dr. Herschel has particularly taken notice of such 
appearances at two or three different times. In 1787, he saw three 
of these volcanoes at once in the dark part of the Moon's disk ; 
two of which were barely visible, or almost extinct; the third 
showed an actual eruption of fire, or of luminous matter. The 
next night Dr. Herschel saw the volcano burn with greater vio- 
lence than on the preceding evening. He considered the eruption 
as resembling a small piece of burning charcoal, when it is covered 
by a very thin coat of white ashes, which frequently adheres to it, 
when it has been some time ignited ; and it had a degree of bright- 
ness, about as strong as that with which such a coal would be 
seen to glow in faint day-light. 

Of Eclipses. 

CCCXX. An Eclipse of the Moon happens 
when the Earth, passing between the Sun and 
Moon, casts its shadow on the Moon ; and, of 
course, the Moon can only be eclipsed at the full, 
or in opposition, and when, at the full, it is in or 
near one of its nodes. 

Obs. 1. For it is only when the Moon is in opposition that it can 
come within the shadow of the Earth, which must always be on 
that side of the Earth which is from the Sun. 

2. The Earth being in the plane of the ecliptic, the centre of its 
shadow is always in that plane ; if therefore the Moon be in its 
nodes, that is. in the plane of the ecliptic, the shadow of the F arth 
will fall upon it ; also, since this shadow is of considerable breadth, 
it is partly above and partly below the plane of the ecliptic ; if 
therefore the Moon in opposition is so near one of its nodes, that 
its latitude is less than half the breadth of the shadow, it will be 
eclipsed. But because the plane of the Moon*s orbit makes an 
angle of more than 5° with the plane of the ecliptic, it will fre- 
quently have too much latitude at its opposition to come within 
the shadow of the Earth. 

859. What is said of the volcanoes which Dr. Herschel saw in the 
Moon ) 860. What occasions an eclipse of the Moon ? 
22 



254 ' ASTRONOMY. 

FHff. 80. 




If 



3. Let S represent the Sun, Fig. SO., m the Moon between the 
Earth and the Sun, a E G b a portion of the Earth's orbit, e and,/' 
two places on the surface of the Earth. The dark part of the 
Moon's shadow is called the Umbra, and the light part the Pen- 
umbra ; now it is evident that if a spectator be situated in that 
•part of the Earth where the umbra falls, that is between candy, 
there will be a total eclipse of the Sun at that place ; at e and./), 
in the penumbra, there will be a partial eclipse ; and beyond the 
penumbra there will be no eclipse. As the Rartii is not always 
at the same distance from the Moon, if an eclipse should happen 
when the Earth is so far from the Moon that the lines F e and 
C f cross each other before they come to the Earth, a spectator 
situated on the Earth, in a direct line between the centres of the 
Sun and Moon, would see a rinsr of light round the dark body of 
the Moon, called an Annular J&clpse; when this happens there 
can be no total eclipse any where, because the Moon's umbra does 
not reach the Earth. People situated in the penumbra will per- 
ceive a partial eclipse. According to M. de Sejour, an eclipse cam 
never be annular longer than 12 min. 24 sec. nor total longer than- 
7 min. 53 sec. The duration of an eclipse of the Sun can never 
exceed two hours. — Keith, I 

CCCXXI. The Sim being" larger than the 
Earth, the shadow of the Earth is a cone y the 
base of which is on the surface of the Earth r and 
the Moon is eclipsed by a section of the Earth's 
shadow. 

Obs. If the Earth were larger than the Sun, or equal to it, it is 
manifest that its shadow would either perpetually enlarge, or be 
always of the same dimensions; hut. in this case, the superior 
planets would sometimes come within it, and be eclipsed, which 

£61. How is an eclipse of the Moon illustrated by means of a figure 1 



ECLIPSES. 255 

never happens. Therefore the Sun is larger than the Earth, and 
produces a shadow from the Earth of a conical form, which does 
not extend to the orbit of Mars. 

CCCXXIL An eclipse of the Moon is Partial, 
when only a part of its disk is within the shadow 
of the Earth ; it is Total, when all its disk is 
within the shadow ; and it is Central, when the 
Earth's shadow falls upon the centre of the 
Moon's disk. 

Obs. 1. Let S represent the Sun, Fig. 80., EG the Earth, and 
m the Moon in the Earth's umbra, having the Earth between it 
and the Sun ; DEP and HGP the penumbra. Now, the nearer 
any part of the penumbra is to the umbra, the less light it receives 
from the Sun, as is evident from the figure ; and as the Moon 
enters the penumbra before it enters the umbra, it gradually loses 
its light and appears less brilliant. 

2. The duration of an eclipse of the Moon, from its first touch- 
ing the Earth's penumbra to the time of its leaving the penumbra, 
can not exceed five hours and a half. The Moon can not continue 
in the Earth's umbra longer than three hours and three quarters, 
in any eclipse, neither can it be totally eclipsed for a longer period 
than one hour and three quarters. As the Moon is actually de- 
prived of its light during an eclipse, every inhabitant upon the 
face of the Earth, who can see the Moon, will see the eclipse. — 
Keith, 169. 

3. If the latitude of the Moon, at the time of the full, is greater 
than the sum of its own semidiameter added to the semidiameter 
of the Earth's shadow, the Moon will not be eclipsed at the time ; 
if its latitude is less than that sum, but greater than the difference, 
it will be partially eclipsed; and if its latitude is less than the 
difference, it will be totally eclipsed, at the time of the full. The 
Moon is centrally eclipsed, only when in opposition or at the full, 
and in one of its nodes. 

4. The Moon in a total eclipse, is not wholly invisible ; and the 
phenomenon is produced by the refraction and reflection of the rays 
of light falling upon the Earth's atmosphere, tow r ards the shadow, 
and consequently towards the Moon in the shadow. 

CCCXXIII. An Eclipse of the Sun happens 

862. If the Earth were as large as the Sun,— what would be the differ- 
ence in the phenomena which would happen in regard to eclipses 7 
863. What is a Partial eclipse 7 E63. What is a Total eclipse? F65. 
What is the difference between a partial and a total eclipse 1 < v 66. What 
is the consequence when an eclipse happens precisely at the nodes'? 
&57, What is the greatest duration of a lunar eclipse ? 



256 ASTRONOMY. 

when tne Moon, passing between the Sun and 
the Earth intercepts the Sun's light; and the 
Sun can only be eclipsed at the new-moon, or 
when the Moon, at its conjunction, is in or near 
one of its nodes. 

Obs. For unless the Moon is in or near one of its nodes, it can 
not appear in or near the same plane with the Sun ; without which, 
it cau not appear to us to pass over the disk of the Sun. At every 
other part of its orbit, it will have so much northern or southern 
latitude, as to appear either above or below the Sun. If the Moon 
is in one of its nodes, having no latitude, it will cover the whole 
disk of the Sun, and produce >i total eclipse, except when its appa- 
rent diameter is less than that of the Sun ; — if it is near one of its 
nodes, having a small degree of latitude, it will only pass over a 
part of the Sun's disk, or the eclipse will be partial. 

CCCXX1V. In a total eclipse of the Sun, the 
shadow of the Moon falls upon that part of the 
Earth where the eclipse is seen. 

Obs. l. A spectator placed any where in the centre, will not see 
any part of the Sun, because the Moon will intercept all the rays of 
light which come to him directly from the Sun ; and it is manifest 
that, in this situation, the Moon, being an opaque body, will cast its 
shadow upon the part of the Earth where tne eclipse is total. 

2. The shadow of the Moon, like that of the Earth, is a cone ter- 
minated in a point ; and the diameter of the Moon's shadow, where 
it falls upon the Earth, is equal to the difference between the appa- 
rent diameters of the Moon and Sun. 

CCCXXV. In a partial eclipse of the Sun, a 
penumbra, or imperfect shadow of the Moon, 
falls upon that part of the Earth where the par- 
tialeclipse is seen. 

Obs. 1. The Moon's penumbra is an increasing cone, its dark- 
ness increasing towards the umbra of the Moon ; and the angle 
of the penumbra is equal to the angle of the Sun's apparent dia- 
meter. The diameter of the penumbra, in that part through which 
. in an eclipse of the Sun, is equal to the sum of 
the apparent diameters of the Sun and Moon. 

Mj . '. I ve positions of the ?un, Moon, and Earth, when 

an eclipse of the Suntakps place I 669. What occasions a solar eclipse 1 
870. w hat circumstances are necessary to produce a Total eclipse of the 
Bun ; and what to produce a Partial eclipse? 



ECLIPSES. 257 

2. If the latitude of the Moon, at the time of the change or con- 
junction, is greater than the sum of the semidiameter of the pen- 
umbra and the Moon's horizontal parallax, there will be no eclipse 
of the Sun at that time ; if less than that suim, there will he an 
eclipse, either partial, or totaL 

CCCXXVL If the Moon, at the time of con- 
junction, is in one of its nodes, the eclipse of the 
Sun will he central 

Ohs. 1. For then the centres of the Earth, Sun, and Moon, being 
-all in the plane of the ecliptic, the centre of the Moon will pass 
-between the Sun's centre and that of the Earth ; and thus the Sun 
will be centrally eclipsed in the zenith, at some place on the sur- 
face of the Earth. 

2. The penumbra of the Moon ; in a central eclipse does not 
cover the whole disk of the Earth. For, the semidiameter of the 
Moon's penumbra, being equal to the sum of the apparent semi- 
diameter of the Sun and Moon, that is, about 16' 2'' -f- 15' 45"', or 
32' at a mean ; its diameter is about 64', whereas the diameter of 
the Earth's disk is about 120' ; whence the penumbra can not 
cover the whole disk. 

3. The height or extent of the shadow of the Moon is about 59 
-semidiameters of the Earth. The semi-angles of the Earth's 
shadow and of the Moon's shadow, being each equal to the Sun's 
•apparent semidiameter, the angles are equal to each other, and 
these cones are similar. Therefore, as the semidiameter of the 

of the Earth's shadow, that is. of the Earth, is to the semi- 
-diameter of the base of the Moon's shadow, that is. of the Moon ; 
so is the height of the Earth's shadow to the height of the Moon's 
shadow. Now, the semidiameter of the Earth being to that of the 
Moon as 11 to 3, and the height of the Earth's shadow about 216 
diameters of the Earth ; hence the height of the Moon's shadow 
is equal to about 59 semidiameters of the Earth; for 11 : 3:;216 ; 
59 nearly. 

CCCXXYII. An eclipse of the Sun is said to 
be Annular, if, at the time of the eclipse, a ring 
of the Sun appears round the edges of the Moon ; 
and a central eclipse of the Sun will be an annu- 
lar one, if the distance of the Moon from the 
Earth at the time of the eclipse, be greater than 
its mean distance. 



871 What circumstances concur in producing a central eclipse of the 
Sun ? 



22* 






258 ASTRONOMY. 

CCCXXVIII. If the orbit of the Earth and 
that of the Moon were both in the same plane, 
there would be an eclipse of the Sun at every 
new-moon ; and an eclipse of the Moon at every 
full-moon. But the orbit of the Moon makes an 
angle of about 5° 9', with the plane of the orbit 
of the Earth, and crosses it in two points called 
nodes. 

Obs. 1. Astronomers have calculated that, if the Moon is less 
than 17° 12' from either node, at the time of new-moon, the Sun 
may be eclipsed ; or if less than 11° 34' from either node, at the 
full-moon, the Moon may be eclipsed ; at. all other times there can 
be no eclipse, for the shadow of the Moon will fall either above 
or below the EarU at the time of new-moon ; and the shadow of 
thft Earth will fell either above or below the Moon, at the time of 
full-moon.* 

2. To illustrate this, suppose the part ed of the Moon's orbit, 
Fig. SO, to be elevated above the plane of the paper or Earth's 
orbit; it is evident that the Earth's shadow, at full-moon, would 
fall below the Moon ; the other part of the Moon's orbit at the 
same time would be depressed below the plane of the paper; and 
the shadow of the Moon, at the time ot new-moon, would fall 
below the Earth. In this case, the Moon's nodes would be 
between E and a, and between G and b, and there would be no 
eclipse, either at the full or new-moon ; but, if the part of the 
Moon's orbit between G and b be elevated above the plane of the 
paper, or Earth's orbit, the part between E and a will be depressed, 
the line of the Moon's nodes will then pass through the centre of 
the Earth and that of the Moon, and an eclipse will ensue. 

3. An eclipse of the Sun begins on the western side of its disk, 
and ends on the eastern ; and an eclipse of the Mcon begins on the 
eastern side of the disk, and ends on the western. 

4. The average number of eclipses in a year is, four, two of the 
Suu, and two of the Moon ; and, as the Sun and Moon areas long 

8.' 2. As the Moon passes between the Sun and the Earth, and as the 
Earth passes between the Sun and Moon, once every month, why do we 
not have a lunar and solar eclipse every month 1 873. flow is this illus- 
trated by a diagram 1 874. What are the Solar and Lunar Ecliptic 
Limits / 875. On which side of the disk does an eclipse begin? 878. 
What is the average number of solar and lunar eclipses in a yearl 

' II is Hn or Moon may be eclipsed, &c. ; 

forthe least distan node, at which the Sun or Moon must- be 

eclipsed, are I iy. These numbers, 17° 12* , 

15o 19', and 11^ ^4', 9o 51', are called the Solar and Lunar Ecliptic 
Limits. 






EPCLIPSES. 259 

below the horizon of any particular place as they are above it. the 
average number of visible eclipses in a year is two. one of the Sun 
and one of the Moon. — Keith. 

It has been found that the greatest number of eclipses^ which, 
can take place in any year is seven, and that the least number is 
tico. When there are seren eclipses in a jear.Jwe are of the Sun, 
and two of the Moon. When there are only two, they are both of 
the Sun. When there are six eclipses in a year, three of them 
will sometimes be of the Sun, and three of the Moon ; and some- 
times ./bur of the Sun. and two of the Moon. Threeis the greatest 
number of eclipses of the Moon that can happen in any one year. 
In every year, there are at least tico eclipses of the Sun. 

CCCXXIX. When the Moon is near the first 
of Aries, and is moving towards the tropic of 
Cancer, the time of its rising will vary but little 
for several days together, and produce the phe- 
nomena of a Harvest-moon. 

Obs. l. If the Moon were to move in the equinoctial, its motion 
in its orbit, by which it describes a revolution, in respect of the 
Sun. in 29 days 12 hours, would carry it every day eastward from 
the Sun about 12° 12'. whence, its time of rising would vary daily 
about 50 minutes. But. because the Moon's orbit is oblique to 
the equator, or equinoctial, nearly coinciding with the ecliptic, 
different parts of it make different angles with the horizon, as 
they rise or set ; those parts which rise with the smaller angles, 
setting with the greatest, and the reverse. Now the less this 
angle is, the greater portion of the orbit rises in the same time. 
Consequently, when the Moon is in those parts which rise or set 
with the smallest angles, it rises or sets with the least difference 
of time, and the reverse. Eut in northern latitudes, the smallest 
angle of the ecliptic and horizon is made when Aries rises and 
Libra sets ; and therefore, when the Moon rises in Aries, it rises 
with the least difference of time. Now the Moon is in conjunc- 
tion in or near Aries, when the Sun is in or near Libra, that is, in 
the autumnal months ; when the Moon rises in Aries, whilst the 
Sun is setting in Libra, the time of its rising is observed to vary 
only two hours in 6 days, in the latitude of London. This is cailecl 
the Harvest-moan. 

2. This circumstance takes place every month ; but as it does 
not happen at the time of full-moon, no notice is taken of it. 
When the Moon's right ascension is equal to six signs, that is, 
when the Moon is in or about the beginning of Libra, there is the 
greatest difference of the times of rising, that is to say, about an 

877. What is the phenomena of a Harvest-moon ; and how is it pro- 
duced ? 



I 



260 ASTRONOMY. 

bour and 15 minutes. Those si^ns which rise with the least 
angle set with the greatest, and the contrary ; therefore, when 
tLere is the least difference in the times of rising, there is the 
greatest in setting, and vice versa. All this may be pleasingly 
exemplified by means of a celestial globe. 

Of the satellites of Jupiter, Saturn, and, Uranus. 

CCCXXX. These satellites, but more particu- 
larly the four satellites of Jupiter, have been 
observed by astronomers with great assiduity 
during the last sixty or seventy years, and the 
tables of their motions have been brought to a 
degree of perfection which the most sanguine 
expectations of astronomers could scarcely have 
anticipated. 

Obs. 1. The following table exhibits the distances of the satel- 
lites from their primaries, their periodical times, according to the 
latest observations; and the angles under which their orbits are 
seen from the Earth, at its mean distance from Jupiter. 





Diameters 


Distances 


Periodical rev 


olu- 




Satellites 


compared 


from their 


tions round their 


Angles subtended 


or Moons. 


with the 


primaries 




primaries 




by their orbits. 




Earth's. 


in miles. 


Days. 


hrs. 


win. 


sec. 




Earth's 


0.272 


236,400 


27 


7 


43 


n§ 




& f 1st. 


1.034 


264,490 


1 


18 


27 


33 


3' 55" 


o J 2d. 


0.813 


420,815 


3 


13 


13 


42 


6 14 


al 3d. 


1.024 


671,234 


7 


3 


42 


33 


9 58 


= [4th. 


0.773 


1,180,532 


16 


16 


31 


50 


17 30 




1st. 




107,000 





22 


37 


36 






2d. 




135,000 


1 


8 


53 


9 




JP 


3d. 




170,000 


1 


21 


18 


26 




!=<! 4th. 




217,000 


2 


17 


44 


51 




S 1 sth. 




303,000 


4 


12 


25 


11 




r /2 | 6th. 




704,000 


15 


22 


41 


14 




1 7th. 




2,050,000 


79 


7 


54 


37 


When discovered. 




fist. 




221,155 


5 


21 


25 


28 


January 18, 1798 


■J 


2d. 




290,821 


8 


16 


57 


47| 


January 11, 1787. 


1 < 


3d. 




339,052 


10 


23 


3 


59 


March 26,1794. 


5 


4th. 




3SS,718 


13 


10 


56 


30 


January 11, 1787. 


£ 


5th. 




777,437 31 


1 


48 





February 9, 1790. 




k 6th. 




1,555,872| 107 


16 


39 


56 


Feb'ary 23, 1794. 



SATELLITES. 261 

2. The third satellite of Jupiter is the largest of all ; the first 
and fourth are nearly of the same size ; and the second is the 
smallest, that is in reference to their apparent magnitudes. 

3. These satellites of Jupiter are of great use in Astronomy — 
in determining the distance of Jupiter from the Earth, at any 
given time ; in affording a ready method of demonstrating that 
the motion of light is progre sive, and not instantaneous, as was 
once supposed ; — and in ascertaining the longitudes of different 
places, by means of the eclipses of those satellites, as explained 
m article 279. 

4. The sixth satellite of Saturn may be seen without much dif- 
ficulty. It was called the Huygenian satellite, from having been 
discovered by Huygens, in the year 16C5. The 3d. 4th, 5th, 
and 7th, were afterwards discovered by Cissini ; the 7lh, in 1671; 
the 5th, in 1672 ; and the 3d and 4th, in 1684. The orbits of the 
first six coincide nearly with the plane of the ring; that of the 7th 
makes a less angle with the ecliptic. 

5. Tables expressing the times at which the eclipses of Jupi- 
ter's moons take place, are calculated for the mean distance of 
that planet from the Earth. From comparing the apparent time 
of these eclipses with the mean time expressed in the tables, it 
appears that when Jupiter is nearest to the Earth, the eclipses 
happen 8 minutes 1\ seconds sooner, and when farthest from the 
Earth, 8 minutes 7h seconds later, than should appear from the 
mean time expressed in the tables. Hence the lime in which 
the rays of light move across the Earth's orbit, the diameter of 
which is 190 million of miles, is found to be 16 minutes 15 seconds ; 
and a ray of light is about 8 minutes 7h seconds in coming from 
the Sun to the Earth. The velocity of light is therefore about 
195,000 miles in a second of time, which is o-reaterthan any other 
motion with which we are acquainted. This grand discovery of 
the progressive motion of light, was made M. Roemer, a Danish 
astronomer, in the year 1674; and has been subsequently con- 
firmed and completely established by means of other observations, 
particularly those made by Dr. Bradley upon the aberration of 
light from the fixed stars.* 



Of Comets, 
CCCXXXI. Comets are solid, opaque, planet- 

878. What is said of the uses of Jup : ter's satell'tes ? < c 79. How is the 
velocity of light determined ? 88 ). When, and by whom, was the grand 
discovery made concerning the progressive motion of light? 

From these observations it is focnd that the velocity of star-light is 
such as carries it through a space equal to the distance of the isua from 
the Earth, in 8 minutes 13 seconds. 



262 ASTRONOMY. 

ary bodies, revolving about the Sun in elliptical 
orbits of immense circumference. 

Obs. 1. The motions of the comets are very complicated ; their 
orbits, instead of being nearly circular, like those of the planets, 
are remarkably long and eccentric. Some of the comets approach 
much nearer to the Sun than the nearest planet — so much so, as 
to be hid in the solar rays ; again, they recede to so groat a dis- 
tance from th*dt luminary as to be carried far beyond the bounds of 
the planetary system, where they become wholly invisible, and 
seem entirely lost from the system. After a lapse of ages, they 
are sometimes again observed, in the planetary regions. 

2. The comets appear in different parts of the heavens and 
move in various directions. They differ, as the stars do, in mag- 
nitude and brightness ; and they are principally distinguished 
from the planets by a luminous train or blaze, called a Tail ; or 
by some hairy or nebulous appearance ; and by their always dis- 
appearing after having been visible during a short time only. 

A comet, at a given distance from the Earth, shines brighter 
when it is on the same side of the Earth with the Sun, than when 
it is on the opposite, side ; from which it appears that the bright- 
ness of a comet depends upon the influence of the Sun. 

3. Though the comets have, in all ages, attracted much atten- 
tion by their appearance in the heavens, and have been regarded 
as objects of terror and superstition ; still they are comparatively 
but little known, scarcely any thing of importance having been 
ascertained resDecling them until the revival of learning and 
science, in the sixteenth century, when particular attention began 
to be paid to Astronomy. We "are as yet totally ignorant of the 
number and of the uses of the comets in the solar system, or in the 
fabric of the world, if indeed they have any use whatever. Nor 
is it to be expected that their number will soon be ascertained, as 
some of them are whole centuries before they make their re-ap- 
pearance. The number of comets that have" been seen, and are 
recorded, appears to be very considerable. Ricciolus, an Italian 
astronomer and mathematician, makes mention in his writings, of 
about 154, which had appeared previous to the year 1651 ; but 
Lubienietz reckons 415 until the year 1665. The Chinese astro- 
nomical books record the appearance of 200 or 300 comets. 

4. Of all the comets which are supposed to exist, though lately 
discovered to be much more numerous than has generally been 
supposed, the periods of only four or Jive have been ascertained 
with any degree of certainty ; though the elements of more than 
100 have been calculated. 

£81. What arc the Comets ? P82. What is said of the motions of co- 
mets and the phenomena exhibited by them 1 8 3. How is a cornet 
described 1 - •,. From what circumstance is it inferred that the bright- 
ness of a comet depends upon the influence of the sauhl 885. What is 
nitherto known respecting the number and uses of the comets 1 



COMETS. 263 

The first comet whose period of revolution has been determined 
with certainty, is that of the year 1652, which had been already 
observed in 1607, by Kepler and Longomontanus, and in 1531, by 
Peter Apian, and which re-appeared in 1759, according to the pre- 
diction of Dr. Halley. The period of this comet is about 76 
years; so that its return may be expected again in 1834 or 1835. 
An other comet that appeare'd in the year 1264 is supposed to be 
the same with that which appeared in 1556; if so, its periodic 
time is about 292 years, and consequently it may be expected to 
return in 1S4S. 

The appearance of one comet has been several times recorded 
m history. This is the celebrated comet of the year 1 680, which, 
from the" accounts of various authors, and from the circumstances 
which attended its apparition, seems to be the same with that 
which appeared in the year 44 B. C, or at the time of Julius 
Caesar, and in the years 531 and 1106 of the Christian era ; also 
in 619 and 2340 before the birth of Christ. As there is an interval 
of 575 years between all these periods except the last. Dr. Halley 
was therefore led to consider the comet of 1680 as the one that had 
appeared at the times above-mentioned, that its period is about 575 
years, and that it will not again make its appearance until the year 
2254. This comet, when nearest to the Sun in 16S0. was only one- 
sixth part of the solar diameter distant from the surface of the Sun ; 
and when farthest, its distance exceeds 138 times the distance of 
the Sun from the Earth ; and according to Dr. Halley, it is 22.412 
times farther from the Sun, when in its aphelion, than in its perihe- 
lion, its greatest distance from the Sun being not less than 13,000 
millions of miles. According to Newton, the velocity of this comet, 
when in the part of its orbit which is nearest to the Sun, is at the 
rate of 8S0.000 miles an hour ; but according to Squire, it is not less 
than 1,240.000 miles an hour; and Newton calculated, that while in 
this part of its orbit, it was exposed to a degree of heat 2000 times 
greater than that of red hot iron. 

5. The periodic returns of three other comets are supposed to be 
ascertained. One of these has a period of about 20 years ; an other, 
known as Encke's comet, revolves round the Sun once in about 1204 
days, and the third, generally designated the Comet of Biela, has a 
periodic time about double that of Encke's comet. 

6. The tails of comets are sometimes of a most prodigious 
length ; that of the comet of 1680. which subtended at Paris an 
angle of 62°, and at Constantinople one of 90°, was at least 100 
millions of miles in length. Various opinions have been enter- 
tained by different philosophers respecting the cause of these extra- 
ordinary appendages, called tails. Some have conjectured the tail 
of a comet to be smoke rising from the body of the comet in a line 
opposite to the Sun. Others regard them as being composed of 
vapours elevated to a considerable height fcy the violence of the 

8-86. How many comets are known to return at certain periods? 687. 
What account is given of each of those comets'? 



264 ASTRONOMY. 

beat to winch they nre exposed in their near approaches to the Sun. 
Dr. Hamilton considers the tails of comets, the aurora borealis, and 
the electric fluid, to be matter of the same kind. Sir Richard Phi- 
lips published in the Monthly Magazine the opinion, that this won- 
derful appendage of comets, is occasioned by the refraction, and 
consequent condensation of the Sun's light thiough the dense atmo- 
sphere of the comet ; — hence the tail is always in an exact straight 
line opposite to the Sun ; and hence, on the principle of a convex 
lens, the tail lengthens as it approaches the Sun, and shortens as it 
departs. 

Of the Parallaxes, Distances, and Magnitudes^ 
of the Heavenly Bodies. 

CCCXXXII. The Parallax of the heavenly 
bodies, is the change of their apparent situation 
with respect to each other, as the observer views 
them from different stations on the Earth, or from 
different parts of the Earth's orbit. 

CCCXXXIII. The Diurnal Parallax is the 
distance between the apparent place of a heavenly 
body, as viewed from the surface of the Earth, 
and its apparent place, considered as viewed from 
the centre of the Earth. 

Obs. 1. Let DAB in Fig. 81, be the Earth, Cfits centre, A the 
station of a spectator on the surface of the Earth; and F, G, H, 
different places of the Moon, or any other heavenly body; TO, 
NM, LI, are its different parallaxes, and THO, or AHC ; MGN, 
or AGO, &c. angles of parallax. 

2. If a spectator in his first station at A, Fig. 81, sees a planet 
at G, its apparent place in the heavens will be N; if now, by the 
diurnal rotation' oi the Earth, he comes into the station P, the 
planet will appear at M, which is the place in which it would 
nave appeared if viewed from C the centre ; thus, in all cases, the 
parallax which arises from the diurnal motion, is the same which 
would arise from a change of station from the surface to the cen- 
tre ; for, in either case, the change of the spectator's line of view 
is the same. Hence appears the propriety of the above definition 
of the diurnal parallax. 

888. What is said concerning those remarkable .-tppendafrps to comets, 
called Tails; and how have they been accounted for? 889. What is 
meant by the term Parallax ? 890. What is the Diurnal Parallax 1 



PARALLAXES, &C. 
Fig. 81. 



265 




CCCXXXIV. The parallax of any planet is 
always proportional to the angle which a semi- 
diameter of the Earth, drawn from the station of 
the spectator upon the surface to the centre, would 
subtend, if viewed from the planet. 

Obs. 1. If the planet be at H, Fig. 81, and the spectator at A, 
AHT will be his line of view ; on changing the station of the 
spectator from A to C, the line of view will become CHO ; whence 
TO will be the parallax. But TO subtends and is proportional 
to THO, or AHO, the angle which the Earth's semidiameter 
would subtend, if viewed from the planet H. 

2. The parallax of a planet depresses its apparent place, by the 
parallactic arc. Thus, if the planet be viewed from C, Fig. 81, 
its apparent place is O ; if from A. its apparent place is T, farther 
from Z the vertex, than O, by the parallactic arc TO. 

3. When the altitude of a body is observed, it must be corrected 
by parallax or refraction, adding the former, and subtracting the 
latter, in order to get the true altitude, or the altitude above the 
rational horizon at the centre of the Earth. 

CCCXXXV. The diurnal parallax of any 
planet, at a given distance from the Earth, is 

F91. How is the parallax illustrated by reference to a diagram ? £92. 
How is the parallax of a planet estimated? 693. In computing the alti- 
tude, of a celestial body, how must the correction by parallax and refrac- 
tion be applied ? 

23 



266 ASTRONOMY. 

i tost when the planet is in the horizon, and 
it decreases as the altitude of the planet increases. 

Obs. 1. The parallax is proportional to the angle which AC, 
FXff. 81. would .subtend, if seen from the planet H ; but this given 
line, viewed from the given distance of the planet, would con- 
tinually diminish in its apparent magnitude, as the degree of 
obliquity at which it is viewed increases, that is, as the planet 
advances from H towards E ; therefore, the parallax is greatest 
in the horizon, and decreases as the planet approaches the vertex. 
The parallactic angle AGO is less than AHC, and AFC less 
than AGC. 

2. The Moon's mean horizontal parallax is 57' 42". 

3. At the same altitude of different planets, their diurnal paral- 
laxes are inversely as their distances from the centre of the Earth, 
because the angles subtended by the semidiameter will be mostly 
as the distances. * 

CCCXXXVI. To measure the distance of the 
Moon from the Earth. 

Def. The Moon's Horizontal Parallax is the angle which a 
semidiameter of the Earth, would subtend, if viewed directly from 
the Moon. 

lllus. Let H be the Moon in the horizon observed by a spectator 
at A, Fig. 81, and C the centre of the Earth. In the triangle 
AHC, let the angle AHC, the Moon's horizontal parallax, be 
found. The angle HAC is a right-angle, and AC the semidiameter 
of the Earth, is known to be 3960 miles. — Hence, AC the sine of 
AHC, 57' 42" is to 3960, as H, taken as a radius, to the number 
of miles in HC, the Moon's distance from the Earth. The Moon's 
mean distance is thus found to be 236,400 miles. 

Obs. According to M. de Lalande, the horizontal semidiameter 
of the Moon, is to its horizontal parallax for the mean radius of 
the Earth as 15' is to 54' 57". 4, or very nearly as 3 to 11 ; hence, 
the semidiameter of the Moon is three-elevenths the radius of 
the Earth. And as the magnitudes of spherical bodies are as the 
cubes of their radii, the magnitude of trie Moon is to that of the 
Earth as 3 3 to ll 3 , that is, as 1 : 49. 

CCCXXXVII. To determine the relative dis- 
tances of tliQ inferior planets from the Sun. 

Elus. If the elongation of Venus, or the greatest angle of Ve- 
nus's distance, be found by observation ; then, as a radius is to the 

894. When is the diurnal parallax of a celestial body greatest 1 895. 
How may the distance of the Moon, or of any other planet from the 
Earth be ascertained ? 



PARALLAXES, &C. 267 

sine of the angle, so is the distance of the Earth to the distance of 
Venus. If the Sun's distance from the Earth be supposed to be 
divided into 1000 equal parts, then the distance of Mercury will in 
this manner be found to be 337, and that of Venus 723. 

CCCXXXVIXI. To determine the relative dis- 
tances of the superior planets from the Sun. 

Obs. If the angle apparently performed by a superior planet, 
while the Earth is moving from one end to the other of its orbit, 
be determined by observation, then half that angle is the angle 
formed by the distance of the Earth from the Sun, seen from the 
planet.* Hence there is given that angle, the right-angle at the 
Sun. and the complement to 1S0°, three angles and the base 1000, 
the Earth's distance, to find the perpendicular in a right-angled 
triangle. 

CCCXXXIX. To find the parallax of the Sun 
by the transit of Venus. 

Obs. 1. This is one of the most important problems known in 
Astronomy; because, when the precise angle is known under which 
the semidiameter of the Earth is seen at the Sun, a triangle is 
given of which one angle is 90° at the Earth's centre, the other 
the parallax at the Sun, being the angle under which the Earth's 
semidiameter is seen, and the other the complement of the same 
to 90°, and the base is the Earth's semidiameter known by mea- 
surement. 

2. Venus would be seen like a dark spot on the disk of the Sun, 
as often as the planet passes in its orbit between the Sun and the 
Earth, but that the plane of its orbit does not coincide with the 
plane of the Earth's orbit, and it passes in regard to the eye of a 
spectator at the Earth, above or below the Sun, except only when 
the planet happens to be in or near its nodes at the time of the 
conjunction. This, however, happens but seldom, the nearest 
transits to our time being in 1639, 1761, 1769, and 1874. 

3. Nothing is more easy to be understood than the application 
of the phenomenon of a transit to ascertain the distance of the 
Earth from Venus and the Sun. All that is wanted, is the angle 
subtended at Venus, by any known portion of the Earth's surface, 
and this is determined by observing the exact time at which Venus 

896. How are the relative distances of the planets from the Sun deter- 
mined 1 



* The principle here explained is altogether erroneous. The angle to 
be measured is that formed at the Earth by two lines drawn from the 
Earth to the Sun and to the planet; and then after the planet has per- 
formed one revolution, the angle subtended at the Earth by the distance 
between the Sun and planet ; and from these data, the calculation may 
be deduced. 



208 ASTRONOMY. 

enters or leaves the Sun's disk at the two places, and then after 
allowing for geographical difference, converting the difference of 
time into degrees, minutes, and seconds ; which is, of course, the 
parallax of Venus. Such is the general principle ; but the details 
of the calculations are rendered complex by the various distances 
of observers, by their opportunities of observing, and by the com- 
pound motion of the Earth and Venus, in the intervals of observa- 
tions. All these points are, however, susceptible of unerring 
calculation. 

4. The parallax of Venus being thus determined, the Sun's pa- 
rallax is easily ascertained by the proportions indicated in Article 
335, and it appears to be 8". 65, that is, the semidiameter of the 
Earth equal to 3960 miles, subtends at the Sun an angle of nearly 
8-j seconds; and by trigonometrical calculation, the perpendicular 
of the tiiangle,or the line joining the centres of the Sun and Earth, 
is 94,530,000 miles. Thus, by logarithms, we have 5.621914 = the 
sine of 8". 65 : 10.000000 :: 3.597695 = log. of 3960 : 94,580,000 
n:iles. But some astronomers reckon tlTe parallax somewhat 
greater, add consequently the distance of the Sun but 93 or 94 
millions of miles, while others reckon the parallax less, and the 
distance 96 millions. 

CCCXL. To measure the distance of any 
planet from the San. 

Mus. Because the real distances of the planets from the Sun 
are as their proportional distances ; as the proportional distance 
of the Earth from the Sun is to the proportional distance of any 
other planet from the Sun, so is the real distance of the Earth 
from the Sun in miles, to the real distance of any planet from the 
Sun in miles. 

In this manner the distances of the planets from the Sun may- 
be found and expressed in miles, as in the several tables of the 
elements of the planets. 

CCCXLI. To find the periodical time of a 
planet. 

Obs. 1. Because, whilst any planet is performing its revolution, 
the Earth is carried forward in its orbit, the planet, after one 
greatest elongation, must not only complete a revolution, but like- 
wise the whole angular space which the Earth has described in 
that time, before the planet arrives a^ain at the same elongation. 
Thus, before Venus can return to the same elongation, besides 
performing an entire revolution in its orbit, equal to .four right- 
angles, it must pass through as much more angular space as the 

897. II m is tip' parallax of the Sun, and thence its distance from the 
Earth, determined bv the transit of Venus I $98. II >w is this illustrated 

by an example 7 809. What is the rule for measuring the distance of any 
planet from the Sun / 






CAUSES OP THE PLANETARY MOTIONS. 269 

Earth has done in the meantime. Hence, as the angular motion 
of Venus is to the angular motion of the Earth in the time between 
the greatest elongation and its return ; so is the periodical time 
of the Earth to the periodical time of Venus. In this manner the 
periodical times of all the planets may be found. Or, they may 
fee found after the manner described under Article 290. 

2. Because the squares of the periodical times of the planets 
were found by Kepler to be as the cubes of their distances, the 
periodical times of any two planets being known, and the com- 
parative or real distance of one of them from the Sun being given, 
the distance of the other may from this proportion be found, and 
conversely. 

Causes of the Planetary Motions, 

CCCXLII. According- to the Greeks, and the 
demonstrations of Newton, the planets are re- 
tained in their orbits by gravitation, which 
draws or impels them towards the centre of 
motion, and carried forward by a projectile force, 
which tends to carry them off at right-angles to 
the other force, or in a tangent to their orbit. 

Fdus. 1. The motion of the primary planets is very simple and 
.uniform, being compounded only of a projectile motion forwards 
in a straight line, which is a tangent to the orbit ; and a gravita- 
tion towards the Sun in the focus. The power which occasions 
the former is called a centrifugal force, and that which occasions 
the latter a centripetal force, and though gravitation is mutual 
between all the planets, being directly as the quantities of matter 
they contain, and inversely as the squares of their distances from 
each other ; yet the motions of the planets are not much affected 
by it ; for their quantities of matter are but very small when com- 
pared with that of the Sun, and therefore its attraction, or their 
gravitation towards it, nearly destroys that of the planets one 
towards an other. 



Fig. 82. 




2. Suppose P, Fig. 82, a planet 
at rest, acted upon by a force which 
tends to carry it towards G, which 
force is directed towards a centre, 
and called the gravitating force ; 
hut at the same instant an other 
force, acting at right-angles to the 
former, tends to carry it towards F ; 
the two forces will ojf course, by the 
laws of mechanical forces, carry it 
23* 



270 ASTRONOMY. 

from P to C, in the diagonal of a square. If then the distances 
PP and PG are supposed to be infinitely small, or to represent 

the powers acting in the smallest conceivable portion of time, 
then the diagonal PC may he conceived to be an infinitely small 
portion of a circle, or a chord of a circle so small that it coincides 
with a circle. 

Fig. 83. 
p -p 3. If then the action of the 

q „„_ v _q same forces be renewed at C 

IN* in the directional CG towards 

• \ ; the centre, and from CP at 

\v right-angles, the planet will 

x. j be turned into the diagonal 

\v i CC, forming an other small 

Q-C^ .J^0C? portion, chord, or increment 

of a circle, and the repetition 
of these impulses continued 
without ceasing, will, of ne- 
- % F cessity,carry the planet round 
.£) the central body in a circular 
orbit. 

Obs. 1. The cause of gravi- 
tation or of projectile forces, 
is not affected to be explained 
by the Newtonian philoso- 
phy ; it is simply assumed, that such forces act according to cer- 
tain laws, let their causes be what they may ; and it is certainly 
more safe to treat of effects than of causes, though to investigate 
the latter is the proper business of Philosophy. 

CCCXLIII. If a body, by a uniform motion, 
describe one side of a parallelogram, in the same 
time that it would describe the adjacent side by 
an accelerated force, this body, by the joint 
action of these forces, would describe a curve 
terminating in the opposite angle of the parallel- 



ed How is the periodical time of a planet ascertained ? <K)1. What is 
the cane of the curvilinear motion <>t the celestial bodies ? 902. Since 
gravitation is mutual between all the planets, why are their motions 
scarcely affecte'd by their mutual attraction? 903. Illustrate or explain 
the manner in which a body proceeds hi a curvilinear direction. 904. 
If a body, by a uniform motion, describes one side of a parallelogram, in 
the same time that it would describe the adjacent side by an accelerating 
force, what effect would the joint action of these forces have upon the 
body? 










CAUSES OF THE PLANETARY MOTIONS. 271 

Let A B C D, Fig. 84, be a parallel- 
ogram, and suppose the body A to be 
carried through A B by a uniform force, 
in the same time that it would be car- 
ried through AC by an accelerated 
force, then by the joint action of these 
forces, the body would describe a curve 
AGID. For, by the preceding illus- 
tration, if the spaces AE,EK,andKB, 
be proportioned to each other, the spaces AF, FH. and HC, will 
be in the same proportion, and the line AGID will be a straight 
line when the body is acted upon by uniform forces ; but in this 
example,_the force' in the direction AB being uniform, would cause 
the body to move over equal spaces, AB, EK, and KB, in equal 
portions of time ; while the accelerative force in the direction 
AC, would cause the body to describe spaces A F. F H, and H C, 
increasing in magnitude in equal successive portions of time ; 
hence the parallelograms AEGF, AKIH, &c. are not about the 
same diagonal, therefore AGID is not a straight line but a curve. 

CCCXLIV. The curvilinear motions of all 
the planets, arise from the uniform projectile 
motion of bodies in straight lines, and the uni- 
versal power called attraction, which draws them 
off from these lines. 

Fig. 85.- 

HIus. 1 . If the body E, Fig. 
85, be projected along the line 
EAF, where it meets with no 
resistance, and is not drawn 
aside by any other force, it 
will, by the laws of motion, 
go on for ever in the same 
direction, and with the same 
velocity. For,theforce which 
moves it from E to A in a giv- 
en time, will carry it from A 
to F in a successive and equal 
portion of time, and so on ; 
there being nothing either to 
obstruct or alter its moiion. 
But, if when the projectile force has carried the body to A, an 
other body, as S, begins to attract it, with a power duly adjusted 

905. How is the composition of equable and accelerated motion illus- 
trated by a diagram 1 906. Whence do the curvilinear motions of all the. 
planetary bodies arise 1 907. How is this illustrated ; and to what does 
the illustration correspond J 




272 



ASTRONOMY. 



and perpendicular to its motion at A, it will be drawn from the 
straight line EAF, and revolve about S in the circle AGOOA.* 
When the body E arrives at O, or any other part of its orbit, if the 
small body M, within the sphere of E's attraction, be projected as 
in the straight line J//?, with a force perpendicular to the attrac- 
tion of E, it will go round the body E, in the orbit m, and accom- 
pany E in its whole course round the body S. — Here S may 
represent the Sun, E the Earth, and M the Moon. 

2. If the Earth at A be attracted toward the Sun at S, so as to 
fall from A to H by the force of gravity alone, in the same time 
which the projectile force singly would have carried it from A to 
F ; by the combined action ot these forces it will describe the 
.curve A G ; and if the velocity with which E is projected from A, be 
such as it would have acquired by falling from A to V, the half of 
AS, by the force of gravity alone, it will revolve round S in a circle. 

CCCXLV. If one body revolve round an other, 
as the Earth round the Sun, so as to vary its dis- 
tance from the centre of motion, the projectile and 
centripetal forces must each be variable, and the 
path of the revolving body will differ from a circle. 



11. Fig.m. If while 
a projectile force car- 
ry E from A to F, the 
Sun's attraction at S 
would bring it from 
A to H, the gravitat- 
ing power would be 
too "Teat for the pro- 
^jectile force; thepla- 
net,therefore,instead 
of proceeding in the 
circle ABC, as in the 
preceding problem, 
would describe the 
curve AO, and ap- 
proach nearer to the 
Sun ; SO being less 
than SA. Now, as 
the centripetal force, 
or gravitating pow- 
er, always increases 
as the square of 
the planet's distance 




If any body revolve round an other in a circle, the revolving body 



CAUSES OF THE PLANETARY MOTIONS. 273 

from the Sun diminishes, when the planet arrives at 3 the centri- 
petal force will be increased, which will likewise increase the 
velocity of the planet, and accelerate its motion from O to P : so 
as to cause it to describe the arches OP, PQ, QR, RD, DT, TV, 
successively increasing in magnitude, in equal portions of time. 

The planet being thus accelerated, it gains such a centrifugal 
force, or tendency to fly ofTat V, in the line of TW, as overcomes 
the Sun's attraction ; this centrifugal or projectile force being too 
great to allow the planet to approach nearer the Sun than it is at 
V, or even to move round the Sun in the circle tabcd, &c. it flies 
off in the curve XZMA, with a velocity decreasing Lis gradually 
from V to A, as if it had returned through the arches VT, TD, DR", 
&c. to A, with the same velocity with which it passed through 
these arches in its motion from A towards V. At A, the planet will 
have acquired the < ame velocity as it had at first, and thus, by the 
varied centrifugal and centripetal forces ; it will continue to move 
' round S. 

But if the action of gravity be too great for the projectile force 
at O, why does it not draw the planet to S ? and if the projectile 
force at V be too great for the centripetal force, or gravity, at the 
same point, whvdoes it not carry the planet farther and farther 
from the Sun, till it is beyond its attraction ? 

First, If the projectile force at A were such as to carry the 
planet from A to G, double the distance, in the same time that it 
was carried from A to F, it would require four times as much 
gravity to retain its orbit ; that is, it must fall through A I in the 
time that the projectile force would carry it from A" to G, other- 
wise it would not describe the curve AOP. But an increase of 
gravity gives the planet an increase of velocity, and an increase 
of velocity increases the projectile force ; therefore, the tendency 
of the planet to fly off from the curve in a tangent P m, is greater 
at P than at O, and greater at Q. than at P, and so on ; hence, 
while the gravitating power increases, the projectile power in- 
creases, so that the planet or comet can not be drawn into the Sun. 

Secondly, The projectile force is the greatest at, or near the 
point V, and the gravitating power is likewise the greatest at that 
point. For if A S be double of V S, the centripetal force at V 
will be four times as great as at A. being as the square of the 
distance from the Sun. If the projectile force at V be double of 
what it was at A, the space VW, which is the double of AF, will 

908. When one body revolves round an oiher. so as to vary its distance 
from the centre of motion, what is the relative effect of the projectile and 
centripetal forces ; and what will be the figure of the orbit described by 
the revolving body 1 909. How are these particulars explained and illus- 
trated in a diagram? 



must be projected with a velocity, equal to that which it would have 
acquired, by falling through half the radius of the circle towards the 
attracting body.— Emerson's Cent. Forces. Prob. U. 



274 ASTRONOMY. 

be described in the same time that AP was described, and the 
planet will be at X in that time. Now if the action of gravity 
had been an exact counterbalance for the projectile force during 
the time mentioned, the planet would have been at / instead of X; 
and it would describe the circle t, a, b, c, &c. ; but the projectile 
force being too powerful for the centripetal force, the planet recedes 
from the Sun at S, and aseends in the curve, YZ3VI, &c. Yet it 
can not fly off" in a tangent in its ascent, because its velocity is 
retarded, and consequently its projectile force is diminished, by 
the action of gravity. Thus, when the planet arrives at Z, its 
tendency to -fly off in a tangent Z n, is just as- much retarded, by 
the action of gravity, as its motion was accelerated thereby at Q, 
therefore it must be retained in its orbit. 

Of the Fixed Stars. 

CCCXLVI. Those luminous points, or shin- 
ing bodies, which always appear in the heavens 
at the same distance from each other, are called 
Fixed Stars; because they do not appear to 
have any proper motion of their own. 

Obs. 1. The fixed stars are luminous bodies. Because they 
appear as points of small magnitude, when viewed through a 
telescope, tney must be at such immense distances as to be invisi- 
ble to the naked eye if they borrowed their light ; as is the case 
with respect to the satellites of Jupiter and Saturn, although they 
appear of very distinguishable magnitude through a telescope. 
Besides, from the weakness of reflected light, there can be no 
doubt that the fixed stars shine with their own light. They are 
easily known from the planets, by their twinkling. 

2. The number of stars, visible at any one time to the naked 
eye, is about 1000 ; but Dr. Herschel, by his skilful and magnifi- 
.cent improvements of the reflecting telescope, has discovered that 
the whole number is great beyond all conception. — The compar- 
ative brightness of the stars is, Sirius 1.00, Canopus .98, Centauri 
.96, Acherni .94, etc. 

3. The magnitudes of the fixed stars appear to be different from 
each other, which difference may arise either from a diversity in 
their real magnitudes, or distances ; or from both these causes 
acting conjointly. The difference in the apparent magnitude of 

910. What (•(]•. <■( takes place when the centripetal force, or action of 
gravity, is too ereal Cor the projectile force; and xiceversa? 911. Which 
of the celestial bodies arc. named Fired stars; and why are the j so 
called I M2. How is it proved that the fixed stars are luminous bodies) 
013. What is the number of stars visible at any one time to the naked 
eyel 



FIXED STARS AND CONSTELLATIONS. 275 

the stars is such as to admit of their being divided into six classes, 
the largest being called stars of the first magnitude, and the least, 
which are visible to the naked eye, stars of the sixth magnitude. 
Stars only visible by the help of glasses, are called Telescopic 
Stars. Dr. Halley very justly remarks, that the stars must be 
infinite in number to maintain their equilibrium in space. And 
Dr. Herschel thinks he has seen stars 42.000 times as far off as 
Sirius. In one instance a cluster of 5000 stars, in a mass, was 
barely visible in the 40 feet telescope, and consequently must have 
been 11 trillions of miles off! 

4. It must not be inferred that all the stars of each class appear 
exactly of the same magnitude ; there being great latitude given 
in this respect ; even those of the first magnitude appear almost 
all different in lustre and size. There are also other stars of in- 
termediate magnitudes, which, as astronomers can not refer to 
any one class, they, therefore, place them between two. Procyon, 
for instance, which Ptolemy reckons of the first magnitude, and 
Tycho of the second, Flamstead lays down as between the first 
and second. So that, instead of six magnitudes, we may say that 
there are almost as many orders of stars, as there are stars ; such 
considerable varieties being observable in their magnitude, colour, 

.and brightness. 

5. To the bare eye, the stars appear of some sensible magnitude, 
owing to the glare of light arising from the numberless reflections 
of the rays in coming to the eye ; this leads us to imagine that 
stars are much larger than they would appear, if we saw them only 
by the few rays which come directly from them, so as to enter the 
eye without being intermixed with others.— Examine a fixed star 
of the first magnitude through a long and narrow tube, which, 
though it takes in as much of the sky as w r ould hold a thousand 
such stars, scarcely renders that one visible. 

6. There seems but litile reason to expect that the real magni- 
tudes of the fixed stars will ever be discovered with certainty, we 
must, therefore, be contented with an approximation, deduced from 
their parallax, if this should ever be ascertained, and the quantity 
of light they afford us compared with that of the Sun. To this 
purpose, Dr. Herschel informs us, that, with a magnifying power 
of 0450, and by means of his new micrometer, he found the appa- 
rent diameter of a Lyrae, to be 0".335, or the third of a second. 

7. The ingenious observations of Kepler upon the magnitudes 
and distances of the fixed stars, deserve to be introduced here, and 
the more so as he has been followed in the conjecture by the great 
Dr. Halley. He observes that there can be only 13 points upon 
the surface of a sphere as far distant from each other as from the 
centre ; and supposing the nearest fixed stars to be as far from 
each other as from the Sun, he concludes there can be only 13 

S14. How are the stars distinguished with regard to their apparent 
magnitudes'? 915. What general name is applied to those stars which 
are visible only through telescopes 7 



276 ASTRONOMY. 

stars of the first magnitude. Hence, at twice that distance from 
the Sun there may be placed four times as many, or 52; at three 
times that distance, nine times as many, or 1 17 ; and soon. These 
numbers will gire pretty nearly the number of stars of the first, 
second, third, &x. magnitudes.* Dr. Halley farther remarks, that 
if the number of stars be finite, and occupy only a part of space, 
the outward stars would be continually attracted to those within, 
and in time would unite into one. But if the number is infinite, 
and they occupy an infinite space, all the parts would be nearly in 
equilibrlo, and, consequently, each fixed star being drawn in oppo- 
site directions would keep its place or move on till it had found an 
equilibrium. 

CCCXLVII. The ancients, that they might 
the better distinguish the stars with regard to 
their situation in the heavens, divided them into 
several constellations, that is, masses or clusters 
of stars, each mass consisting of such as are near 
to each other. And to distinguish these groups 
or systems from each other, they gave them the 
names of such men or things as they fancied the 
space they took up in the heavens represented. 
To these, several new constellations have been 
added by modern astronomers. 

Obs. 1. The ideal delineations of those figures of animals and 
other objects, which include the constellations or asterisms, are 
dispersed all over the heavens, and a particular situation is as- 
signed to each, as may be seen upon a common celestial globe, or 
upon a planisphere or map of the heavens ; yet some spaces in 
the heavens remained here and there, which, according to the 
ancient distribution of the stars were out of the bounds of the con- 
tiguous constellations. The stars which were included in those 
spaces were called Unformed Stars; but, as represented on the 
modern celestial globes, the constellations are made to comprehend 
all the unformed or extra-constellated stars. % 

916. What is said <>f the distance and number of the fixed stars? 917. 
What reason have we to suppose that the number of stars is infinite 1 
918. \\ mstellation? 919. What is the use of distributing 

Che fixed stars iptc constellations 7 920. What is meant by Unformed, 
Sta r.v ? 



* This statement appears to be erroneous, for, in this case, the whole 
number of stars visible to the naked eye from all sides of the Earth, 
would not much exceed 1^'J, the sum of the above number 






FIXED STARS AND CONSTELLATIONS. 277 

2. Besides the names of the constellations, the ancient Greeks 
crave particular names to some single stars, or small collections of 
stars. Thus, the cluster of small stars in the neck -of Taurus, the 
Bull, was called the Pie ades ; five stars in the Bull's Face, the 
Hyades; a bright star in the Breast of Leo. the Lion's Heart, or 
Cor Leonis ; and a iarge star between the Knees of Ecoies.Arctu- 
rus, &c. Several of the brishtest fixed stars have also particular 
names, as Sirius. Aldebaran. Regains, Castor. Procyon, Alioth.&c. 

3. As it would be an endless Task to give a proper name to each 
star, astronomers, in order that the memory may not be burdened 
with a multiplicity of name?, denominate the stars of each con- 
stellation by means of the letters of the Greek alphabet, which 
are applied to them according to their apparent relative size. 
The principal or brightest star in the constellation is designated 
by a, AlpJia ; the ne^t in brightness, by J. Beta ; the third, by y, 
Gamma; and so on. When the number of stars in a constella- 
tion, exceeds the letters in the Greek alphabet, the letters of the 
Roman alphabet, a. b, c. d. &c. are applied to the remaining stars 
in the same manner ; and when these are not sufficient, the num- 
bers. 1. 2. 3. 4. 5. &c. are used to designate the rest in the same 
regular succession ; so that by these means, the stars may be rea- 
dily known and spoken of with as much ease as if each had a 
separate, name.* 

4. The celestial sphere is usually divided into three portions, 
the zodi c. and the riortheiTi and southern hemispheres, or more 
properly the two regions to the north and south of the zodiac. 
Astronomers have accordingly divided the constellations into three 
classes, called the northern, the southern, and the zodiarai. The 
number of northern constellations is 37, of the southern 47. and of 
the zodiacal 12 ; making 96 in the whole. The ancient astrono- 
mers reckoned only 48 constellations, — 12 in the zodiac. 21 to the 
north, and 15 to the south. Modern astronomers, however, by 
curtailing several of the ancient constellations of some of their 
stars, which they distributed into new constellations, and bv ar- 
ranging into constellations the unformed stars, or those which 
were between the ancient constellations, have increased their 
number to 96. as above stated. 

The term zodiac is derived from a Greek word signifying an 

921. Are all the stars included in the several constellations ! 902. How 

are the several stars in each const-Hat on distinguish? d ! 923. How have 
astronomers contrived to be understood, when speakin? of any parti- 
cular star in a constellation ? 



This method of designating the stars in each constellation bv the 
Greek and Roman alphabets was the invention of John Bayer, a German 
lawyer and astronomer, who introduced it into his ia. or 

ts of the constellations, first publshed in 160"\ in folia But in our 

opinion, why not avoid all this perplexity and confusion of Greek. Ro- 
man, and numerical characters, bv adopting at once the nur 
characters 7 

24 



278 ASTRONOMY. 

animal, because most of the constellations in that zone, which are 
twelve in number, are represented by the outlines of the figures of 
animals; as, Ar.es, the Ram; Taurus, the Hull; Gemini, the 
Twins; C.inrn\ the Crab; Leo, the Lion; Virgo, the Virgin ; 
Libra, the Balance ; Scorpio, the Scorpion ; Sagittarius, the 
Archer; Capricornus, the Goat; Aquarius, the Water-bearer; 
and Pisces, the Fishes. 

5. The luminous part of the heavens, called the M Iky -Way, 
consists of fixed stars too small to be seen by the naked eye. In 
a paper on the constructions of the heavens, Dr. Herschel says, 
" It is very probable, that the great stratum called the Milky-Way, 
is that in which the Sun is placed, though perhaps not in the cen- 
tre of its thickness, but not far from the jjlace where some smaller 
stratum branches from it. Such a supposition will satisfactorily, 
and with great simplicity account for all the phenomena of the 
Milky- Way, which, according to this hypothesis, is no other than 
the appearance of the projection of the stars contained in this 
stratum, and its secondary branch." 

6. In an other paper on the same subject, he says,— -" We will 
now retreat to our own retired station in one of the planets attend- 
ing a star in the great combination with numberless others ; and 
in order to investigate what will be the appearances from this con- 
tracted situation, let us begin with the naked eye. The stars of 
the first magnitude, being, in all probability, the nearest, will fur- 
nish us with a step to begin our scale ; setting off, therefore, with 
the distance of Sirius or Arcturus, for instance, as unity, we will 
at present suppose, that those of the second magnitude are at 
double, and those of the third at treble the distance, and so forth- 
Taking it then for granted, that a star of the seventh magnitude 
is about seven times as far from us as one of the first, it follows 
that an observer, who is enclosed in a globular cluster of stars, 
and not far from the centre, will never be able, with the naked 
eye, to see the end of it; for since, according to the above estima- 
tions, he can only extend his view about seven times the distance 
of Sirius, it can not be expected that his eves should reach the 
borders of a cluster, which has perhaps, not less than fifty stars in 
depth every where around him. The whole universe, therefore, 
to him, will be comprised in a set of constellations, richly orna- 
mented with scattered stars of all sizes. Or, if the united bright- 
ness of a neighbouring cluster of stars should, in a remarkably 
clear night, reach his sight, it will put on the appearance of a 
small, faint, nebulous cloud, not to be perceived without the 
Greatest attention. Allowing him the use of a common telescope, 
he begins to suspect, that all the milkiness of the bright path 
which surrounds the sphere may be owing to stars. By increasing 
his power of vision, he becomes certain", that the Milky-Way is, 

924. What is it that produces the luminous appearance called the 
XUhy-Way 7 



FIXED STARS AND CONSTELLATIONS. 279 

indeed, no other than a collection of very small stars, and the 
nebula nothing but clusters of stars." 

7. Dr. Herschel then solves a general problem for computing 
the length of the visual ray ; — that of the telescope, which he used, 
will reach to stars 497 times the distance of Sirius. Now Sirius 
can not be nearer than 100.000 X 190.000.000 miles, therefore Dr. 
Herschel's telescope will at least reach, to 100.000 X 190.000.000 
X 497 miles.* And Dr. Herschel says, that in the most crowded 
part of the Milky- Way he has had fields of view that contained 
no less than 588 stars, and these were continued for many minutes, 
so that in a quarter of an hour, he has seen 116,000 stars pass 
through the field view of a telescope of only 15' aperture ; and at 
an other time, in 41 minutes, he saw 258,000 stars pass through 
the field of his telescope. Every improvement in his telescope 
has discovered stars not seen before, so that there appear no 
bounds to their number, or to the extent of the universe. 

8. There are spots in the heavens, called Nebula, some of which 
consist of clusters of telescopic stars ; others appear as luminous 
spots of different forms. The most considerable one is that which 
is about midway between the two stars on the Blade of Orion's 
Sword, marked d by Bayer, and discovered in the year 1656 by 
Huygens. It consists only of 7 stars ; and the other part is a 
bright spot upon a dark ground, and appears like an opening into 
brighter regions beyond. Dr. Halley and others have discovered 
nebula in different parts of the heavens. In the Connoisance des 
Temps, for 1783 and 17S4, there is given a catalogue of 103 nebula 
observed bv Le Messier and M. Mechain. But to Dr. Herschel 
we are indebted for catalogues of .2000 nebula and clusters of stars, 
which he himself had discovered. Some of them form a round 
compact system ; others are more irregular, of various forms ; and 
some are long and narrow. The appearance of luminous spaces 
in the heavens, SirJlichard Philips denies to arise from light per 
se ; but ascribes the luminosity of all such spaces to the multitude 
of planets, asteroids, satellites, and cometary bodies, with which 
those spaces are filled. 

9. New stars sometimes appear, while others disappear. Seve- 
ral stars, mentioned by the ancient astronomers, are not now to be 
found ; several are now visible to the naked eye, which are not 
mentioned in the ancient catalogues ; and some stars have sud- 
denly appeared, and again, after a considerable interval, vanished; 
also a change of place has been observed in some stars. 

10. From an attentive examination of the stars with good teles- 

925. What are those spots, called Nebula, which are seen in different 
parts of the heavens 1 926. What is said concerning the appearance of 
new starsi 

* This part of the observation seems to be quite exceptionable, and 
inconsistent with Dr. Herschel's own observations elsewhere on t!*e 
&auie subject. 



2S0 ASTRONOMY. 

^copes, many which appear only single to the naked eye, are found 
to consist of numerous stars, a Ilrrculis, is a double star, so is 
n Bootes ; and Dr. Herschel, by his highly improved telescopes, 
has found about 700. 

From a series of observations on double stars, Dr. Herschel has 
found that a great many of them have changed their situations 
with regard to each other; that the one performs a revolution 
round the other ; and that the motion of some is direct, while that 
of others is retrograde. He has observed that there is a change 
in more than 50 of the double stars, either in the distance of the 
two stars, or in the angle made by a line joining them with the 
direction of their daily motion. 

Dr. Herschel has observed that the smaller of the two stars 
composing Castor, has a revolution of 342^ years round the other; 
the double star y Leonis has a period of 1200 years ; e Bootes, of 
1681 years ; 6 Serpentis. of 375 years ; y Virginis, of 708 years ; 
and so of the rest ; — but the life of one man is evidently too short 
to attain correct results, in regard to periods so disproportionate 
to his narrow space of existence. 

11. These motions of the stars amonsr themselves being appa- 
rent to observation, the doctrine of Dr. Herschel and other astro- 
nomers is rendered probable, that the Sun has a motion or orbit of 
its own among the fixed stars of the Milky-Way, at the rate of the 
Earth's motion, carrying with it all the planets, just as the planets 
themselves, carry with them their systems of satellites in their 
own orbits. The rotation of the Sun on its inclined axis, accord- 
ing to the theory of Sir Richard Philips, seems to indicate the action 
of a centrifugal force in the Sun, and to render the notion, that the 
whole solar system is analogous to a primary and its satellites, ex- 
ceedingly provable. 

In a paper on the Construction of the Heavens, Dr. Herschel 
has the following observation ; — " That the Milky-Way is a most 
extensive stratum of stars of various sizes, admits no longer of 
the least doubt ; and that our Sun is actually one of the heavenly 
bodies belonging to it, is as evident." 

12. In February, 1814, Dr. Herschel, the prince of astronomers, 
read to the Royal Society, the results of thirty years' observations 
on nebula, with the best telescopes ever possessed by man. He 
conceives that the stars form independent systems among them- 
selves. He considers our Sun as part of that shoal or system 
which we call the Milky-Way, and that all the stars of the first, 
second, and third magnitude, belong to that vast cluster. The 
stars, he remarks, are not spread in equal portions over the celes- 
tial sphere, but are found in patches, each containing many thou- 
sands, and many more than the eye can separate from the mass. 
These he calls Clusters ; and he conceives they have a constant 

■ 1 c tncerning double stars; their revolutions round 

each other ; and their proper motions in general 1 128. What inference 
is drawn from the proper motions of the fixed stars 1 



FIXED STARS AND CONSTELLATIONS. 



281 



disposition to unite more closely, by a power, which he calls the 
Clustering Power ; doubtless the same power which is describee! 
in the observation to article 342. He gives an account of eighty 
of these clusters, some of the drawings of which are copied here. 
The following figure represents a compressed cluster of 
6tars, the centre part 8' long, 2' broad. 

Fig. 87. 

f* % **i £+>£ fet 






The following figure represents an other similar cluster op 
stars. 









i^£\ 88. 



# 



* * * * 






■a 

* *-? ***** 



929. What is the hypothesis of Dr. Hersehel, respecting the " construc- 
tion of the heavens V 



24* 



282 ASTRONOMY. 

The following- figure represents a globular cluster 2* in 
diameter, as seen with Herschel's 40 feet telescope. 

Fig. 89. 

UW&U& CCCXLVIII. The Annu- 

^t al Parallax of a heavenly 

T^V ; bod y, is the change of its ap- 

$$#* '*&$& parent place, as it is viewed 

>; from the Earth in its annual 




; motion; and is the angle 
which the diameter of the 
orbit would sub- 
tend, if that diameter could 
be viewed from that body. 
CCCXLIX. If the distance of an object is 
greater than 400,000 times the base, the angles- 
at the stations do not sensibly differ from right- 
angles ; consequently, the lines drawn from the 
object to the stations, are physically parallel, and 
the parallax of an object, the distance of which 
is above 400,000 times greater than that between 
the two stations of observation, is consequently 
insensible. 

Obs. If the object is at a greater distance from either station 
than 400,000 times the base, the angle at one of the stations being 
90°, the angle at the other will be more than 89° 59' 57". 9, the 
difference of which angle and 90° being scarcely more than 2", is 
too small to become sensible by observation. 

CCCL. If the parallax of an object, observed 
with an instrument sufficiently exact to measure 
an angle of 2", is insensible, the distance of the 
object from either station can not be less than 
400,000 times the base, yet it may be greater in 
assignable ratio. 

930. What is the Annual Parallax? 931. What are the principles 
npun which the annual parallax of a celestial body is ascertained 1 



FIXED STARS AND CONSTELLATIONS. 283 

Obs. Lines drawn from any given points in a base, to an object, 
may be esteemed in practice, parallel, without any sensible error, 
jf the distance of the object be more than 100.000 times the base. 
Rays, therefore, diverging from any point in the Sun's disk upon 
the surface of the Earth, may be esteemed parallel, it their dis- 
tance from each other does not exceed about 070 miles at the 
Earth's surface ; because 970 is to the distance of the Earth from 
the Sun in a proportion of 1 to 100.000. 

CCCLI. The fixed stars have no sensible 
annual parallax, because when the place of any- 
star is observed by the best instruments, from 
opposite points of the Earth's orbit, its apparent 
place in the heavens remains the same ; which 
could not be the case, if the angle of its parallax 
were so much as two seconds. 

Obs. 1 . Hence it appears that the fixed stars are so remote, that 
a diameter of the Earth s orbit bears no sensible proportion to their 
distance ; or that a diameter of the Earth s orbit, if viewed from 
one of the fixed stars, would appear as a point. 

2. The distance of a star must be greater than 100.000 times 
the base, from the extremities of which it is observed : that is, 
greater than 100.000 times the diameter of the orhit of the Earth, 
or greater than 100.000 X 190.000,000, which is 19 billions of miles 
as the least possible distance of the nearest fixed star. 

3. The parallax of a fixed star, being not more thau 2'', the Sun, 
when view T ed from that star, would appear under an an°:le less than 

39' Q. i 7 

~ '''_ , or less than — -, and, therefore, could not be distinguished 
200.000' _ 100' J . ' s 

from a point. 

4. Since bodies, equal in magnitude and splendour to the Sun, 
being placed at the distance of Fhe fixed stars, would appear to us 
as the fixed stars now do, it may be supposed probable, that the 
fixed stars are bodies similar to the Sun, which is the centre to the 
solar system. This being the case, the reason will appear, why 
a fixed star, when viewed through a telescope magnifying 200 
times, appears no other than a point. For the apparent diameter 
of the star being less than one-hundredth part of a second, when 
magnified 200 times, will subtend an angle less than 2", at the eye 
of the spectator observing it in the telescope. 

932. Why have not the stars any sensible parallax 1 



INDEX. 



Accelerated Motion, 

Acoustic Tubes, 

Acoustics, - 

Adhesion, 

Air, - 

— its Weight, 

its Elasticity, - 

Air-gun, 
Air-pump, - 
Altitude, 
Amplitude, - 
Analysis, 
Anemometer, 
Angle of Incidence, 

of Reflection, 

Anomaly, 

Mean, 

True/ 

Antarctic Circle, 
Antecedentia, 
Antipodes, - 
Antaeci, - 
Aphelion, 
Apogee, - 
Apsis, 

Arctic Circle, 
Armature, - 
Asteroids, 
Astronomy, 
Atmosphere, 

Pressure of, 

Attraction, - 
Aurora Boreal is, 

Australis, - 

Axioms of Philosophy, 
Axis, 

of the Earth, 

of a Lens, 

in Peritrochio, - 

Azimuth, - . 



Barometer, 
Battery, Electrical, 
Galvanic, 

-Voltaic, 



Beam of light, 
Belts, Jupiter's, 
Body, 



Page. 
38 
116 
109 

23 

25 

87, 91 

89, 93 

103 

89 

203 

203 

12 

ioo 

123 
123 

224 

221 

225 

202 

199 

213 

213 

223 

224, 247 

223 

- 202 

184 

222, 240 

189 

85 

87, 101 

22 

169 

169 

13 

223 

201 

121 

57 

203 



103 

105 

173 

its power, 175 

172, 178 

118 

241 

15 



Boracite, 
Breast Wheel, 
Burning-glass, - 

Caloric, 
Calorimotor 
Camera Obscura, - 
Capillary Attraction, 

Tubes, - 

Catoptrics, 

Causes of the Celestial MotionSj 

Celestial Bodies, -* 

Divisions of, 

Central Forces, 
Centre of Emanation, 

— of Gravity, 

— of Motion, - 

Ceres, - 
Chemistry, - 
Choroides, 
Chromatics, 
Clustering Power, 
Coats of the Eye, - 
Coelum Empyreum, 
Cohesion, 
Colours. f 

- Primary, - 

Secondary, 

Theory of, 

of Light, 

Comets, 

Compound Motion, 
Concave Glasses, - 
Concord, 
Condenser, 
Conductors, 



Conjunction, 



- Galvanic, « 

- of Sound, 



Consequentia, 
Constellations, - 
Convergency, 
Convex Glasses, 
Copernicus, 
Curvilinear, 

Day, Natural, 



- Inferior, 
• Superior, 



192, 



206, 



Pagb. 

158 

84 

125 

24 
177 
151 
23 
23 
116 
s, 269 
191 
19.1 
41 
29 
45 
46 
239 
12 
141 
116 
281 
131 
194 
22 
135 
136 
136 
139 
136 
261 
36 
14S 
114 
103 
155 
172 
111 
203 
225 
225 
199 
276 
122 
143 
195 
42 

219 



INDEX. 



285 



Day and Night, 234, 

Declination, 

Decomposition of Light, 
Definitions, - 
Difference of Longitude, 
Dimness of Light, - 
1 Coptics, 
Discord, 

Discharging Rod, 
Distance of Planets, 

of Fixed Stars, 

Divergency, 
Divisibility, 
Double Stars, 
Dynamics, 

Earth, 

Orbicular motion of, 

Magnitude of, 

Axis of, - 

■ Orbit of, - 199, 

■ Rotundity of, 

Spheroidal Form of, 

Earthquakes, - 
Eccentricity, 
Echo, - 
Eclipses, 

■ Annular, 

— Central, - 

• Partial, 

Total, 

of the Moon, - 

of the Sun, 

Ecliptic, 
Ecliptic Limits, 
Effects, - 
Elastic Medium, 
Elasticity, 

of Air, - 

Electric, 

Charge, - 

Discharge, 

Fluid, 

Shock, 

Spark, 

Electrical Battery, - 
- Machine, 



Page, i 
212, 215 Electro-Magnetism, - 
203 i Electrometer, 

- 136 l Elements of the Planets, 
201, 223 Elongation^ - 
136, 212 Equation of the Centre, 

133 j of Time, - 

116, 143! Equator, 
115 .Equinoctial, 

- 164 Points, 

224 i Equinoxes, - 

235, 279 | Ethics, - 
122 Eye, - 
17, 20 Eye-glass, 
230 

- 32 Fits of Easy Reflection, &c. 

Fixed Stars, 
199, 229 Fluid, 



Electricity, 



Electricity, 



Electrics, 



Fluidity, cause of, 
Fluids, - 

— Aeriform, 
Pressure of, 



U9 

2\ 

■ 2 1 

2?5, 217 

- 199 
231, 231 

99, 170 

22^ 

113, 114 

253 

- 257 
257 

- 256 
256 

- 253 
255 

202, 253 
259 

- 14 
- 26, 117 

24, F5 
■ 89, 93 

- 155 
15- 

- 163 
153 

- 164 
159, 162 

- 154 
161 

- 159 
153 

Galvanic, - 170 

- Motion of, . 162 
Negative, - 156 j Glottis, 

- Origin of, - 169 j Gravitation, 
-Positive, - 156 Gravitating Force, - 
■Resinous, - 153 Gravity, 
-Vitreous, - 15S — Absolute, - 

155 1 — • Centre of s 



Focal Listance, 
Focus, of Light, 

of an Orbit 

Force, 

Centrifugal, 

Centripetal, 

Gravitating, 

Projectile, 

Forcing Pump, 
Fountain, Artificial 
Fountains, - 
Friction, 
Friction Rollers, 
Fulcrum, 
Full-moon, - 

Galvani, 

Galvanic Battery, • 

Circle, 

Conductors, 

Pile, 

Shock, 

Trough, - 

Galvanism, 

— its Effects on Animals, 

on Metals, 

Geocentric Place, - 
Globe, Artificial, 

— Celestial, - 
Terrestrial, 



•26. 



Page. 

- 178 
161 

- 223 
225 

- 225 
219 

201, 202 
20S 

- 203 
233 

- 11 
131 

145, 147 

129 
192, 274 

- 66 
S3, 66 

- 66 
S5 

- 67 
124 

- 119 
205 

30, 41 
42, 233 
42, 269 
269 
42, 269 
SI 

- 95 
83 

- 64 
65 

- 53 
247 

- 170 
173 

- 173 
172 
173 

- 174 
174 

- 170 
177, 179 
175, 176 

2 3 
231 
201 
201 
114 
26 
269 
26 
72 
45 



286 



INDEX. 



Gymnotus Electricus, 

Hammer-lever, 

Harding, 

Hare, Professor, 

Harmony, 

liar vest Moon, 

Heliocentric Place, 

Herschel, 

Horizon, 

3 (umoura of the Eye, 

Hurricanes, 89, 99, 

Hydraulics, 

Hydrodynamics, 

Hydrogen, - - 

i Gas, 

Hydrometer, 
Hydrostatical Balance, 

: Bellows, 

Paradox, 

Hydrostatics, 
Hygrometer, 
Hyperbola, - 

Incidence, 
Incident Ray, 
Inclined Plane, 
Induction, - 
Inertness, 
In Unity, 
Inflection, 
Instruments, Optical, 

Pneumatic, 

Stringed, 

— Wind, - 

Insulation, - 
Introduction, - 
Invisible Lady, 

Juno, 
Jupiter, 



Satellites of, 



Land-breeze, 

Latitude, 

Laws of Falling Bodies, 

of Fluids, 

of Light, 

of Motion, 

of Revolving Bodies, 

Laws of Vision, 
- 

Leyden Experiment, 

Phial, 

Libration, 
Ligamenta Ciliaria, 



p 


AGE. 
170 




57 


. 


239 




177 


. 


114 




259 


. 


2*3 




244 


. 


2)3 




131 


101, 


170 


66, 


78 




65 




86 


169, 


175 




75 


. 


75 




71 


. 


70 




65 


. 


108 




24 


. 


123 




128 


. 


63 




13 


. 


21 




17 


- 


119 




142 


. 


102 


114, 


115 




115 




156 


. 


11 




116 


. 


239 




24 f) 


- 


241 




98 


202, 


393 




39 


. 


67 




116 


. 


32 




43 
67 


. 




121 


. 


53 




164 


. 


164 




249 


. 


132 J 



Light, 



- Beams of, 

- Colours of, 



of, 



Heterogeneous, 

Homogeneous, - 

Inflected, 

Progressive Motion 

Rays of, 

Reflected, 

Refracted, - 

Lightning, 
Line of Apsides, 

of Direction, 

of Nodes, 

Longitude, 
Luminous Bodies, - 

Magic Lantern, 
Magnet, - 

Armed, 

Artificial, - 

Dipping of, - 

Directive Power of, 

Horse-shoe, - 

Natural, - 

Properties of, 

Strength of, 

Magnetic Declination, - 

Meridian, 

Needle, - 183. 

Variation of; 



Pagb. 
116 

- 118 
136, 138 

136 
136 
119 
120 
118 

- 119 
119 

- 167 

- 224 

- 47 
226 

292, 208 
118, 191 



Magnetism, 



Theory of, 



187. 



Mariner's Compass, 

Mars, 

Matter, 

— Infinite Divisibility of, 19, 20 
Natural Divisions of, 20 



152 
181 
184 
181 
182 
183 
184 
181 
181 
186 
183 
183 
187 
187 
181 
187 
189 
237 
15, 19 



• Properties of, 



Mechanical Poioers, - 

Mechanics, 

Medium, 

Mercury, 

Meridian, 

Metaphysics, 

Microscope, 

: Compound, 

Simple, 

Solar, - 

Water, 

Milky-Way, 

Minimum Visible, 

Mirror, 

Mobility, 

Momentum, 

Monsoons, 

Moon, 



16, 19 

- 51 
• 51, 52 

- 118 
227 

- 2J3 

11 

- 143 
145 

- 144 
145 

- 145 
- 191, 278 

- 134 
142 

- 21 
41 

- 97 
192, 246 



INDEX. 



287 



Moon, Mountains of, 

Orb it of, 

Spots in, 

-Volcanoes in, 



Moons, 
Motion, - 

Causes of, - 

Composition of, 

• Direct, 

Resolution of, 

Retarded. - 

Retrograde, 

Simple, 

Velocity of, 



Multiplying Glass 
Myopes, 

Nadir, 

Natural History, 

Nebula, 

New-moon, 

Node, 

Non-conductors, 

Northern Light, 

Nutation, 

Object Glass, 

Obliquity of the Eclipti 

Octave, 

Opaque Bodies, 

Opposition, - 

Optic Nerve, - 

Optics, 

Orbit, - 

Oscillation, - 

Overshot Wheel. 

Oxygen, 

Pallas, - 
Parabola, 
Paradox, 
Parallax, 



■ Annual, 
Diurnal, - 
Horizontal, 



Pencil of Rays, 

Pendulums, 

Penumbra, - 

Perigee, 

Perihelion, - 

Perioeci, 

Phantasmagoria, 

Phenomena, 

Philosophy, - 

Physical Affections of Matter. 

Physics. - - - 11. 

Physiology, 



Page, i 
251 

- 25 I 
191, 251 

- 253 
192 

- 32 
34 

- 36 
199 

. 36 
39 

- 199 

36 

- 35 
153 

- 134 

233 

- 15 
191, 279 

-' 247 
226 

- 155 
169 

211, 233 

245, 247 

- 210 
115 

IIS, 191 
204 

- 132 
116 

190, 192 
59 

- 85 
86 

- 239 

5) 

70, 2L7, 218 

264 

. 2-2 

264 

-' 266 

119 

- 49 
254 

223. 247 
223 

213, 214 
153 

- 14 
11 
22 
19 
11 



Page. 

Piazzi, 239 

Planets, - - - - 192 

Distances of, - 268 

Motion of, - - 225 

Periodic Time of, 225 

Planetary Motion, - - 269 

Pneumatics, - - 95 

Polar Star, - - - 215 

Poles, - - 153, 176 
Precession of the Equinoxes, 232 

Presbytes, 134 

Pressure and Resistance, - 23 

Primary Planets, - 192, 222 

Primum Mobile, - - 194 

Prism, - ' - - 136 

Prismatic Refraction, - - 137 

Professor Oersted, - - 1 73 

Projectiles, - - - 50 

Pulley, - - - 58 

Pump, Common, - - 81 

Pupil of the Eye, - - 132 

Pyrometer, - - -106 



ICj 



Radiant Point, 
Radius. - 
Radius-Vector, 
Rainbow, 

Artificial, 

Rarefaction, 
Rays, Converging, 

Crossing of, 

Diverging, . 

Parallel, 

Reflected, - 

Refracted, 

Reaction, 

Recession of the Equinoxes ; 

Reflection, - 

Refraction, 

Repulsion, 

Retina, 

Revolution, 



30, 



C6 ; 



- 32, 



Atmospherical, 



Sidereal, 

Synodic, 

■ : Tropical, 

Right Ascension, 
Rules of Reasoning, 

Satellites, 
Saturn, 

Satellites of, ■ 

Sclerotica, - 
Screw, - 
Sea-breeze, • 
Seasons, 
Secondary Planets, ■ 



ri9 

44 
224 
140 
141 

97 
119 
126 
119 
119 
123 
123 

35 

232 

123 

119, 122 

2!8 

25 
131 

42 

2:6 

246 

225, 216 

203 

14 

192 

242 

242 

131 

61 

93 

214 

1S2, 245 



225. 



288 



INDEX. 





Page. 




Page. 


Secular Motion, 


. 


223 


Time, Mean, 


219 


Bight, 


- 


12 


Periodical, 


- 225 


Signs of the Ecliptic, - 




200 


Tornado, 


99 


of the Zodiac, 


202, 


2 6 


'I orpedo. 


- 170 


Silurus Electricus, 




170 


Tourmalin, - 


158 


Bingfng, 


- 


11. 


Trade Winds, - 


- 97 


Solar Svstem, - 


193, 


1 - 


Transits, 


228 


Solidity, 




119 


Transmission, - 


- 118 


Bolstic 


203, 


216 


Transparent Medium, 


118 


Sound, 




109 


Tremella, 


- 146 


Velocity of, 


- 


112 


- 


2 2 


Speaking Trumpet, 


- 


113 


Tunica Cornea, 


- 132 


Specific Gravity, 


- 




Twilight, 


218 


of Men, 






Twinkling of Stars, 


192, 245 


Spectrum, 


- 


137 


Tympanum, 


109 


Speculum, - 


- 


142 






Spots in the Moon, 




237 


Umbra, 


254 


Springs, Intermitting, 




79 


Undershot Wheel, 


- 85 


Stars, 


iro, 


191 


Unison, 


115 


'Evening, 


229, 


241 


Uni verse, 


- 11 


Morning, - 


229, 


211 


Uranus, 


243 


Clysters of, - 


- 


2-1 




- 243 


^ciicniits Oi, 


Stationary, 




225 






Steam Engine, 


- 


f2 


Vacuum, 40, 81, 


90, 92, 1C2 


Sun, 




22 


Vapour, 


26 


— — Diameter of, - 


- 


220 


Velocity of Light, 


- 120 


— — Distance of, 


. 


22'.) 


Venus, 


228 


Rotation of, 




220 


Vernier, 


- 104 


Spots on, - 


- 


22,; 


Vesta, 


238 


Synthesis, - 




12 


Vibration, - 49, 109, 


Syphon, 


. 


89 


Vicissitudes, 


204 


Syringe, 


- 


102 


nf T)iv inr* w*«»v»« oh 






System, - 


n, 


1'4 


Vision, 


117 


Copernican, 


. 


195 


Voice, 


- 114 


Egyptian, 


. 


1'.5 


Voltaic Pile, 


- 173, 178 


of Vortices, 


. 


197 


Voltaism, 


- 170 


Ptolemaic, 




194 






Pvt In ^orcft n 




195 


Wandering Stars, - 


192 


Semi Tychonic 




197 


Water-spout, 

W t-d go, 


99, 170 


Tychonic, - 


. 


1£6 


61 


Syzigies, 


. 


217 


Weight, 


53, 72 






Wheel and Axis, - 


57 


Table of the Effects of Heat, 


K6 


Whirlwind, 


101, 170 


of Specific Gravity, 


. 


77 


Whispering Gallery, 


113 


Tackles, 


. 


6» 


Wind, - 


86, 89, 97 


Tails of Comets, 


192, 








Telescopes, - 




147 


Year, Period icaj, • 


232 


Theorems of Dr. Keill, 




17 


Tropical, 


- 232 


Thermometers, 




r 5 






Thunder, 


. 


167 


Zenith, 


203 


Tides, 


. 


233 


Zodiac, • 


203, 205 


Time, Apparent, 


• 


219 


Zones, 


202 



FINIS. 



